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feat: Add Metrics Hook (#114)
Signed-off-by: André Silva <[email protected]>
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‎.github/component_owners.yml‎

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src/OpenFeature.Contrib.Hooks.Otel:
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- bacherfl
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- toddbaert
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- askpt
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src/OpenFeature.Contrib.Providers.Flagd:
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test/OpenFeature.Contrib.Hooks.Otel.Test:
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- bacherfl
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- toddbaert
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- askpt
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test/OpenFeature.Contrib.Providers.Flagd.Test:
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- matthewelwell
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ignored-authors:
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- renovate-bot
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- renovate-bot

‎.vscode/settings.json‎

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{
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"dotnet.defaultSolution": "DotnetSdkContrib.sln"
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}

‎src/OpenFeature.Contrib.Hooks.Otel/MetricsConstants.cs‎

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namespace OpenFeature.Contrib.Hooks.Otel
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{
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internal static class MetricsConstants
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{
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internal const string ActiveCountName = "feature_flag.evaluation_active_count";
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internal const string RequestsTotalName = "feature_flag.evaluation_requests_total";
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internal const string SuccessTotalName = "feature_flag.evaluation_success_total";
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internal const string ErrorTotalName = "feature_flag.evaluation_error_total";
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internal const string ActiveDescription = "active flag evaluations counter";
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internal const string RequestsDescription = "feature flag evaluation request counter";
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internal const string SuccessDescription = "feature flag evaluation success counter";
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internal const string ErrorDescription = "feature flag evaluation error counter";
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internal const string KeyAttr = "key";
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internal const string ProviderNameAttr = "provider_name";
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internal const string VariantAttr = "variant";
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internal const string ReasonAttr = "reason";
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internal const string ExceptionAttr = "exception";
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}
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}

‎src/OpenFeature.Contrib.Hooks.Otel/MetricsHook.cs‎

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using System;
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using System.Collections.Generic;
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using System.Diagnostics;
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using System.Diagnostics.Metrics;
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using System.Reflection;
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using System.Threading.Tasks;
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using OpenFeature.Model;
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namespace OpenFeature.Contrib.Hooks.Otel
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{
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/// <summary>
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/// Represents a hook for capturing metrics related to flag evaluations.
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/// The meter name is "OpenFeature.Contrib.Hooks.Otel".
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/// </summary>
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public class MetricsHook : Hook
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{
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private static readonly AssemblyName AssemblyName = typeof(MetricsHook).Assembly.GetName();
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private static readonly string InstrumentationName = AssemblyName.Name;
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private static readonly string InstrumentationVersion = AssemblyName.Version?.ToString();
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private readonly UpDownCounter<long> _evaluationActiveUpDownCounter;
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private readonly Counter<long> _evaluationRequestCounter;
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private readonly Counter<long> _evaluationSuccessCounter;
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private readonly Counter<long> _evaluationErrorCounter;
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/// <summary>
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/// Initializes a new instance of the <see cref="MetricsHook"/> class.
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/// </summary>
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public MetricsHook()
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{
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var meter = new Meter(InstrumentationName, InstrumentationVersion);
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_evaluationActiveUpDownCounter = meter.CreateUpDownCounter<long>(MetricsConstants.ActiveCountName, description: MetricsConstants.ActiveDescription);
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_evaluationRequestCounter = meter.CreateCounter<long>(MetricsConstants.RequestsTotalName, "{request}", MetricsConstants.RequestsDescription);
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_evaluationSuccessCounter = meter.CreateCounter<long>(MetricsConstants.SuccessTotalName, "{impression}", MetricsConstants.SuccessDescription);
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_evaluationErrorCounter = meter.CreateCounter<long>(MetricsConstants.ErrorTotalName, description: MetricsConstants.ErrorDescription);
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}
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/// <summary>
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/// Executes before the flag evaluation and captures metrics related to the evaluation.
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/// The metrics are captured in the following order:
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/// 1. The active count is incremented. (feature_flag.evaluation_active_count)
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/// 2. The request count is incremented. (feature_flag.evaluation_requests_total)
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/// </summary>
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/// <typeparam name="T">The type of the flag value.</typeparam>
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/// <param name="context">The hook context.</param>
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/// <param name="hints">The optional hints.</param>
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/// <returns>The evaluation context.</returns>
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public override Task<EvaluationContext> Before<T>(HookContext<T> context, IReadOnlyDictionary<string, object> hints = null)
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{
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var tagList = new TagList
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{
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{ MetricsConstants.KeyAttr, context.FlagKey },
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{ MetricsConstants.ProviderNameAttr, context.ProviderMetadata.Name }
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};
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_evaluationActiveUpDownCounter.Add(1, tagList);
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_evaluationRequestCounter.Add(1, tagList);
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return base.Before(context, hints);
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}
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/// <summary>
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/// Executes after the flag evaluation and captures metrics related to the evaluation.
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/// The metrics are captured in the following order:
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/// 1. The success count is incremented. (feature_flag.evaluation_success_total)
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/// </summary>
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/// <typeparam name="T">The type of the flag value.</typeparam>
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/// <param name="context">The hook context.</param>
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/// <param name="details">The flag evaluation details.</param>
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/// <param name="hints">The optional hints.</param>
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/// <returns>The evaluation context.</returns>
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public override Task After<T>(HookContext<T> context, FlagEvaluationDetails<T> details, IReadOnlyDictionary<string, object> hints = null)
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{
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var tagList = new TagList
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{
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{ MetricsConstants.KeyAttr, context.FlagKey },
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{ MetricsConstants.ProviderNameAttr, context.ProviderMetadata.Name },
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{ MetricsConstants.VariantAttr, details.Variant ?? details.Value?.ToString() },
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{ MetricsConstants.ReasonAttr, details.Reason ?? "UNKNOWN" }
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};
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_evaluationSuccessCounter.Add(1, tagList);
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return base.After(context, details, hints);
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}
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/// <summary>
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/// Executes when an error occurs during flag evaluation and captures metrics related to the error.
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/// The metrics are captured in the following order:
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/// 1. The error count is incremented. (feature_flag.evaluation_error_total)
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/// </summary>
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/// <typeparam name="T">The type of the flag value.</typeparam>
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/// <param name="context">The hook context.</param>
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/// <param name="error">The exception that occurred.</param>
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/// <param name="hints">The optional hints.</param>
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/// <returns>The evaluation context.</returns>
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public override Task Error<T>(HookContext<T> context, Exception error, IReadOnlyDictionary<string, object> hints = null)
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{
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var tagList = new TagList
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{
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{ MetricsConstants.KeyAttr, context.FlagKey },
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{ MetricsConstants.ProviderNameAttr, context.ProviderMetadata.Name },
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{ MetricsConstants.ExceptionAttr, error?.Message ?? "Unknown error" }
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};
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_evaluationErrorCounter.Add(1, tagList);
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return base.Error(context, error, hints);
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}
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/// <summary>
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/// Executes after the flag evaluation is complete and captures metrics related to the evaluation.
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/// The active count is decremented. (feature_flag.evaluation_active_count)
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/// </summary>
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/// <typeparam name="T">The type of the flag value.</typeparam>
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/// <param name="context">The hook context.</param>
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/// <param name="hints">The optional hints.</param>
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/// <returns>The evaluation context.</returns>
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public override Task Finally<T>(HookContext<T> context, IReadOnlyDictionary<string, object> hints = null)
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{
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var tagList = new TagList
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{
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{ MetricsConstants.KeyAttr, context.FlagKey },
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{ MetricsConstants.ProviderNameAttr, context.ProviderMetadata.Name }
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};
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_evaluationActiveUpDownCounter.Add(-1, tagList);
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return base.Finally(context, hints);
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}
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}
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}

‎src/OpenFeature.Contrib.Hooks.Otel/README.md‎

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## Usage
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## Usage - Traces
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For this hook to function correctly a global `TracerProvider` must be set, an example of how to do this can be found below.
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var tracerProvider = Sdk.CreateTracerProviderBuilder()
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.AddSource("my-tracer")
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.ConfigureResource(r => r.AddService("jaeger-test"))
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.AddOtlpExporter(o =>
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.AddOtlpExporter(o =>
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{
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o.ExportProcessorType = ExportProcessorType.Simple;
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})
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![](./assets/otlp-error.png)
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## Usage - Metrics
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For this hook to function correctly a global `MeterProvider` must be set.
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`MetricsHook` performs metric collection by tapping into various hook stages.
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Below are the metrics extracted by this hook and dimensions they carry:
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| Metric key | Description | Unit | Dimensions |
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| -------------------------------------- | ------------------------------- | ------------ | ----------------------------------- |
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| feature_flag.evaluation_requests_total | Number of evaluation requests | {request} | key, provider name |
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| feature_flag.evaluation_success_total | Flag evaluation successes | {impression} | key, provider name, reason, variant |
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| feature_flag.evaluation_error_total | Flag evaluation errors | Counter | key, provider name |
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| feature_flag.evaluation_active_count | Active flag evaluations counter | Counter | key |
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Consider the following code example for usage.
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### Example
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The following example demonstrates the use of the `OpenTelemetry hook` with the `OpenFeature dotnet-sdk`. The metrics are sent to the `console`.
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```csharp
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using OpenFeature.Contrib.Providers.Flagd;
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using OpenFeature;
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using OpenFeature.Contrib.Hooks.Otel;
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using OpenTelemetry;
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using OpenTelemetry.Metrics;
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namespace OpenFeatureTestApp
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{
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class Hello {
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static void Main(string[] args) {
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// set up the OpenTelemetry OTLP exporter
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var meterProvider = Sdk.CreateMeterProviderBuilder()
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.AddMeter("OpenFeature.Contrib.Hooks.Otel")
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.ConfigureResource(r => r.AddService("openfeature-test"))
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.AddConsoleExporter()
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.Build();
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// add the Otel Hook to the OpenFeature instance
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OpenFeature.Api.Instance.AddHooks(new MetricsHook());
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var flagdProvider = new FlagdProvider(new Uri("http://localhost:8013"));
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// Set the flagdProvider as the provider for the OpenFeature SDK
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OpenFeature.Api.Instance.SetProvider(flagdProvider);
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var client = OpenFeature.Api.Instance.GetClient("my-app");
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var val = client.GetBooleanValue("myBoolFlag", false, null);
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// Print the value of the 'myBoolFlag' feature flag
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System.Console.WriteLine(val.Result.ToString());
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}
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}
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}
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```
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After running this example, you should be able to see some metrics being generated into the console.
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## License
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Apache 2.0 - See [LICENSE](./../../LICENSE) for more information.

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