Document how to add custom metrics (#2350)

docs/manual-instrumentation.md

@@ -5,6 +5,8 @@ manual instrumentation. By using both automatic and manual instrumentation, you
 better instrument the logic and functionality of your applications, clients,
 and frameworks.
 
+## Traces
+
 To create your custom traces manually, follow these steps:
 
 1. Add the `System.Diagnostics.DiagnosticSource` dependency to your project:
@@ -39,6 +41,46 @@ You can see a sample console application with manual instrumentation [here](../e
 > Note that an `Activity` created for `NonRegistered.ManualInstrumentations`
 `ActivitySource` is not handled by the OpenTelemetry Automatic Instrumentation.
 
-Further reading:
+## Metrics
+
+To create your custom metrics manually, follow these steps:
+
+1. Add the `System.Diagnostics.DiagnosticSource` dependency to your project:
+
+    ```xml
+    <PackageReference Include="System.Diagnostics.DiagnosticSource" Version="7.0.0" />
+    ```
+
+2. Create a `Meter` instance:
+
+    ```csharp
+    using var meter = new Meter("Examples.Service", "1.0");
+    ```
+
+3. Create an `Instrument`:
+
+    ```csharp
+    var successCounter = meter.CreateCounter<long>("srv.successes.count", description: "Number of successful responses");
+    ```
+
+4. Update the `Instrument` value. Optionally, set tags:
+
+    ```csharp
+    successCounter.Add(1, new KeyValuePair<string, object?>("tagName", "tagValue"));
+    ```
+
+5. Register your `Meter` with OpenTelemetry.AutoInstrumentation by setting the
+    `OTEL_DOTNET_AUTO_METRICS_ADDITIONAL_SOURCES` environment variable:
+
+    ```bash
+    OTEL_DOTNET_AUTO_METRICS_ADDITIONAL_SOURCES=Examples.Service
+    ```
+
+    You can set the value to either `Examples.Service`
+    or to `Examples.*`, which registers the entire prefix.
+
+You can see a sample console application with manual metric instrumentation [here](../examples/demo/Service/Program.cs).
+
+## Further reading
 
 - [OpenTelemetry.io documentation for .NET Manual Instrumentation](https://opentelemetry.io/docs/instrumentation/net/manual/#setting-up-an-activitysource)