[bug] wrong precision for Histogram buckets with float type

[tbolon]

Package

OpenTelemetry.Exporter.OpenTelemetryProtocol

Package Version

Package Name	Version
OpenTelemetry.Exporter.OpenTelemetryProtocol	1.14.0

Runtime Version

net10.0, net472

Description

When using an Histogram for a float type (because second is now the recommended time unit) and giving a custom InstrumentAdvice with float values for HistogramBucketBoundaries, the generated buckets are generated with rounding errors due to float lack of precision.

For example, a bucket with a limit of 0.025f will be emitted with a tag named "le0.02500000037252903".

Note that as a workaround, using double will fix this problem.

Steps to Reproduce

1. Create and start a local otel collector configured with debug traces:

receivers:
  otlp:
    protocols:
      http:
        endpoint: 0.0.0.0:4320

exporters:
  debug:
    verbosity: normal

service:
  pipelines:
    metrics:
      receivers: [otlp]
      exporters: [debug]

2. Create the following program.cs:

#:package OpenTelemetry.Exporter.OpenTelemetryProtocol@1.14.0

using System.Diagnostics.Metrics;
using OpenTelemetry;
using OpenTelemetry.Metrics;
using OpenTelemetry.Resources;

using (Sdk.CreateMeterProviderBuilder().ConfigureResource(r => r.AddService("MyService")).AddMeter("MyApp").AddOtlpExporter(o =>
{
    o.Protocol = OpenTelemetry.Exporter.OtlpExportProtocol.HttpProtobuf;
    o.Endpoint = new Uri("http://localhost:4320/v1/metrics");
}).Build())
{
    var meter = new Meter("MyApp", "1.0");
    var histogram = meter.CreateHistogram<float>(
        "my_histogram",
        advice: new InstrumentAdvice<float>
        {
            HistogramBucketBoundaries = [
                0.001f, 0.005f, 0.01f, 0.025f, 0.05f, 0.1f, 0.25f, 0.5f, 1f, 2.5f, 5f, 10f, 30f, 60f, 120f]
        });

    while (Console.ReadKey().Key != ConsoleKey.Escape)
    {
        var value = Random.Shared.NextSingle() * 10;
        histogram.Record(value);
        Console.WriteLine($"Recorded value: {value}");
    }
}

3. Start the program, press some keys and wait the collector to show some logs.

Expected Result

(newlines added for clarity)

ScopeMetrics #0 MyApp@1.0
my_histogram{} count=34 sum=150.80813431739807 min=0.18132150173187256 max=9.395732879638672
  le0.001=0
  le0.005=0
  le0.01=0
  le0.025=0
  le0.05=0
  le0.10=0
  le0.25=2
  le0.5=2
  le1=3
  le2.5=4
  le5=7
  le10=16
  le30=0
  le60=0
  le120=0
  0
        {"resource": {"service.instance.id": "4f79c7e7-8560-4109-a8a4-9f921e51a1e5", "service.name": "otelcol-contrib", "service.version": "0.142.0"}, "otelcol.component.id": "debug", "otelcol.component.kind": "exporter", "otelcol.signal": "metrics"}

Actual Result

(newlines added for clarity)

ScopeMetrics #0 MyApp@1.0
my_histogram{} count=34 sum=150.80813431739807 min=0.18132150173187256 max=9.395732879638672
  le0.0010000000474974513=0
  le0.004999999888241291=0
  le0.009999999776482582=0
  le0.02500000037252903=0
  le0.05000000074505806=0
  le0.10000000149011612=0
  le0.25=2
  le0.5=2
  le1=3
  le2.5=4
  le5=7
  le10=16
  le30=0
  le60=0
  le120=0
  0
        {"resource": {"service.instance.id": "4f79c7e7-8560-4109-a8a4-9f921e51a1e5", "service.name": "otelcol-contrib", "service.version": "0.142.0"}, "otelcol.component.id": "debug", "otelcol.component.kind": "exporter", "otelcol.signal": "metrics"}

Additional Context

Maybe this behavior should be considered as normal, in this case perhaps it should be documented somewhere that float histogram should be avoided and replaced by double.

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[martincostello]

Seems to me that this is inherent to the use of float, so would be something to clarify in documentation to help users avoid the footgun.

The conversion from one numeric type to double is delegated to the framework:

opentelemetry-dotnet/src/OpenTelemetry/Metrics/MetricStreamIdentity.cs

Line 200 in 75fe387

explicitBucketBoundaries[i] = Convert.ToDouble(adviceExplicitBucketBoundaries[i], CultureInfo.InvariantCulture);