Add metrics configuration docs

Author: rosebyte

docs/core/diagnostics/metrics-collection.md

@@ -2,7 +2,7 @@
 title: Collect metrics - .NET
 description: Tutorial to collect metrics in .NET applications
 ms.topic: tutorial
-ms.date: 10/27/2021
+ms.date: 08/27/2026
 ---
 
 # Collect metrics
@@ -21,6 +21,38 @@ For more information on custom metric instrumentation and options, see [Compare
 
 - [.NET 6.0 SDK](https://dotnet.microsoft.com/download/dotnet) or a later
 
+## Configure metrics with IConfiguration
+
+Starting with .NET 8, Generic Host applications that use the default builder settings automatically call <xref:Microsoft.Extensions.DependencyInjection.MetricsServiceExtensions.AddMetrics*> and load the `Metrics` section from <xref:Microsoft.Extensions.Configuration.IConfiguration>. In a non-hosted dependency injection application, call `AddMetrics` and <xref:Microsoft.Extensions.Diagnostics.Metrics.MetricsBuilderConfigurationExtensions.AddConfiguration(Microsoft.Extensions.Diagnostics.Metrics.IMetricsBuilder,Microsoft.Extensions.Configuration.IConfiguration)> explicitly to bind the same section:
+
+```csharp
+services.AddMetrics(metrics =>
+    metrics.AddConfiguration(configuration.GetSection("Metrics")));
+```
+
+Import the `Microsoft.Extensions.DependencyInjection` and `Microsoft.Extensions.Diagnostics.Metrics` namespaces to use these extension methods.
+
+> [!IMPORTANT]
+> The `Metrics` configuration only selects which instruments registered <xref:Microsoft.Extensions.Diagnostics.Metrics.IMetricsListener> implementations receive. It doesn't register listeners or activate their subscriptions, create metrics, aggregate measurements, export data, or automatically configure OpenTelemetry, Prometheus, `dotnet-counters`, or any other collector.
+
+Use the following sections to choose which meters the rules apply to:
+
+| Section | Applies to |
+| --- | --- |
+| `EnabledMetrics` | Both global and local meters. |
+| `EnabledGlobalMetrics` | Meters created directly with <xref:System.Diagnostics.Metrics.Meter>. |
+| `EnabledLocalMetrics` | Meters created by the service container's <xref:System.Diagnostics.Metrics.IMeterFactory>. |
+
+An `EnabledMetrics`, `EnabledGlobalMetrics`, or `EnabledLocalMetrics` section directly under `Metrics` applies to every registered listener. To target one listener, nest the section under the listener's <xref:Microsoft.Extensions.Diagnostics.Metrics.IMetricsListener.Name> value, such as `Metrics:MyListener:EnabledMetrics`. Listener and instrument names require exact matches without regard to case.
+
+Within any of these sections, a Boolean meter entry enables or disables every instrument whose meter name matches. To set instrument-level rules, use an object with instrument names as keys. The special `Default` entry supplies a fallback. A Boolean `Default` entry at the meter level applies to otherwise unmatched meters. Within a meter object, `Default` applies to otherwise unmatched instruments. A listener doesn't receive an instrument if no rule matches it.
+
+The following `appsettings.json` example enables the `Contoso.Web` meter by default but disables its noisy `request-duration` instrument:
+
+:::code language="json" source="snippets/Metrics/appsettings.json":::
+
+Meter names use case-insensitive prefix matching. A meter name can contain at most one `*` wildcard to match a prefix and suffix. More-specific rules take precedence in this order: listener-specific rules, longer meter-name patterns, instrument-specific rules, and scope-specific rules. If equally specific rules conflict, the last configured rule wins.
+
 ## Create an example app
 
 Before metrics can be collected, measurements must be produced. This tutorial creates an app that has basic metric instrumentation. The .NET runtime also has [various metrics built-in](built-in-metrics.md). For more information about creating new metrics using the <xref:System.Diagnostics.Metrics.Meter?displayProperty=nameWithType> API, see [the instrumentation tutorial](metrics-instrumentation.md).

docs/core/diagnostics/snippets/Metrics/appsettings.json

@@ -0,0 +1,10 @@
+{
+  "Metrics": {
+    "EnabledMetrics": {
+      "Contoso.Web": {
+        "Default": true,
+        "request-duration": false
+      }
+    }
+  }
+}