[Http Client Latency] Add new page describing the HttpClientLatencyExtension methods

docs/core/extensions/httpclient-latency-extensions.md

@@ -0,0 +1,159 @@
+---
+title: Use the HttpClientLatency
+description: Learn how to use the HttpClientLatency with dependency injection in your .NET workloads.
+author: IEvangelist
+ms.author: dapine
+ms.date: 09/29/2025
+---
+
+# HTTP client latency telemetry in .NET
+
+### Get started
+
+When building applications that communicate with other services over HTTP, understanding the performance characteristics
+of those HTTP operations is essential. The `AddHttpClientLatencyTelemetry` extension method provides a way to collect
+detailed timing information about HTTP requests without requiring changes to your application code.
+HTTP client latency telemetry integrates with the existing IHttpClientFactory system to:
+
+* Collect timing data for different stages of HTTP requests
+* Track HTTP protocol information used for requests
+* Measure garbage collection impact during HTTP operations (on .NET platforms that support it)
+* Provide consistent telemetry data for performance analysis
+
+### [.NET CLI](#tab/dotnet-cli)
+
+```dotnetcli
+dotnet add package Microsoft.Extensions.Http.Diagnostics --version 10.0.0
+```
+
+### [PackageReference](#tab/package-reference)
+
+```xml
+<PackageReference Include="Microsoft.Extensions.Http.Diagnostics" Version="10.0.0" />
+```
+
+---
+
+For more information, see [dotnet package add](../tools/dotnet-package-add.md) or [Manage package dependencies in .NET applications](../tools/dependencies.md).
+
+### Register HTTP client latency telemetry
+
+To add HTTP client latency telemetry to your application, call the `AddHttpClientLatencyTelemetry` extension method when configuring your services:
+
+```csharp
+using Microsoft.Extensions.DependencyInjection;
+
+var builder = WebApplication.CreateBuilder(args);
+
+// Add HTTP client factory
+builder.Services.AddHttpClient();
+
+// Add HTTP client latency telemetry
+builder.Services.AddHttpClientLatencyTelemetry();
+```
+
+This registration adds a DelegatingHandler to all HTTP clients created through IHttpClientFactory, collecting detailed latency information for each request.
+
+### Configure telemetry options
+
+You can customize how telemetry is collected by configuring the `HttpClientLatencyTelemetryOptions`:
+
+```csharp
+// Configure with delegate
+builder.Services.AddHttpClientLatencyTelemetry(options => 
+{
+    options.EnableDetailedLatencyBreakdown = true;
+});
+
+// Or configure from configuration
+builder.Services.AddHttpClientLatencyTelemetry(
+builder.Configuration.GetSection("HttpClientTelemetry"));
+```
+
+### Configuration options
+
+The `HttpClientLatencyTelemetryOptions` class offers the following settings:
+
+### Collected telemetry data
+
+When HTTP client latency telemetry is enabled, the following information is collected:
+
+#### Timing checkpoints
+
+Timestamps are recorded for key stages of the HTTP request lifecycle:
+
+* DNS resolution (Http.NameResolutionStart, Http.NameResolutionEnd)
+* Socket connection (Http.SocketConnectStart, Http.SocketConnectEnd)
+* Connection establishment (Http.ConnectionEstablished)
+* Request headers (Http.RequestHeadersStart, Http.RequestHeadersEnd)
+* Request content (Http.RequestContentStart, Http.RequestContentEnd)
+* Response headers (Http.ResponseHeadersStart, Http.ResponseHeadersEnd)
+* Response content (Http.ResponseContentStart, Http.ResponseContentEnd)
+
+#### Measures
+
+On supported platforms:
+
+* Http.GCPauseTime - Records garbage collection pause duration during HTTP operations
+* Http.ConnectionInitiated - Indicates when a new connection is established
+
+#### Tags
+
+* Http.Version - Records the HTTP protocol version used for the request
+
+### Accessing telemetry data
+
+The collected telemetry data can be accessed through the standard `ILatencyContextAccessor`:
+
+```csharp
+public class ApiService
+{
+    private readonly IHttpClientFactory _httpClientFactory;
+    private readonly ILatencyContextAccessor _latencyContextAccessor;
+
+    public ApiService(
+        IHttpClientFactory httpClientFactory,
+        ILatencyContextAccessor latencyContextAccessor)
+    {
+        _httpClientFactory = httpClientFactory;
+        _latencyContextAccessor = latencyContextAccessor;
+    }
+
+    public async Task<object> GetDataAsync()
+    {
+        var client = _httpClientFactory.CreateClient();
+        var response = await client.GetAsync("https://api.example.com/data");
+
+        // Access latency information
+        var context = _latencyContextAccessor.Current;
+        if (context != null)
+        {
+            // Calculate DNS resolution time
+            var dnsStart = context.GetCheckpoint(HttpCheckpoints.NameResolutionStart);
+            var dnsEnd = context.GetCheckpoint(HttpCheckpoints.NameResolutionEnd);
+            if (dnsStart != null && dnsEnd != null)
+            {
+                var dnsTime = dnsEnd.Value - dnsStart.Value;
+                Console.WriteLine($"DNS resolution took: {dnsTime.TotalMilliseconds}ms");
+            }
+
+            // Get HTTP version used
+            var httpVersion = context.GetTag(HttpTags.HttpVersion);
+            if (httpVersion != null)
+            {
+                Console.WriteLine($"HTTP protocol version: {httpVersion}");
+            }
+        }
+
+        return await response.Content.ReadFromJsonAsync<object>();
+    }
+}
+```
+
+### Platform considerations
+
+HTTP client latency telemetry works across all supported .NET platforms with the following considerations:
+
+* Core timing metrics are available on all platforms (.NET 9, .NET 8, .NET Standard 2.0, .NET Framework 4.6.2)
+* Garbage collection metrics (Http.GCPauseTime) are only available on .NET 8 and .NET 9
+* The implementation automatically adapts to the capabilities of the target platform