Doc for Asp.net core enrichment from request & response object (#1193)

* Doc for Asp.net core enrichment from request & response object

* nit

* moin

* trailing space removal

* m

* m

src/OpenTelemetry.Instrumentation.AspNetCore/README.md

@@ -14,6 +14,71 @@ This includes incoming gRPC requests using
 dotnet add package OpenTelemetry.Instrumentation.AspNetCore
 ```
 
+## Enable AspNetCore Instrumentation
+
+`OpenTelemetry.Instrumentation.AspNetCore` must be enabled at application
+startup, typically in the `ConfigureServices` method of your ASP.NET Core
+application's `Startup.cs` class, as shown below.
+
+```csharp
+services.AddOpenTelemetryTracerProvider(
+    (builder) => builder
+                .AddAspNetCoreInstrumentation()
+                );
+```
+
+## Filtering
+
+TODO.
+
+## Enriching automaticaly collected activity with additional information
+
+This instrumentation library stores the raw `HttpRequest`, `HttpResponse`
+objects in the activity. This can be accessed in ActivityProcessors, and
+can be used to further enrich the Activity with additional tags as shown
+below.
+
+```csharp
+internal class MyAspNetCoreEnrichingProcessor : ActivityProcessor
+{
+    public override void OnStart(Activity activity)
+    {
+        // Retrieve the HttpRequest object.
+        var httpRequest = activity.GetCustomProperty("OTel.AspNetCore.Request")
+                          as HttpRequest;
+        if (httpRequest != null)
+        {
+            // Add more tags to the activity
+            activity.SetTag("mycustomtag", httpRequest.Headers["myheader"]);
+        }
+    }
+
+    public override void OnEnd(Activity activity)
+    {
+        // Retrieve the HttpResponse object.
+        var httpResponse = activity.GetCustomProperty("OTel.AspNetCore.Response")
+                           as HttpResponse;
+        if (httpResponse != null)
+        {
+            var statusCode = httpResponse.StatusCode;
+            bool success = statusCode < 400;
+            // Add more tags to the activity or replace an existing tag.
+            activity.SetTag("myCustomSuccess", success);
+        }
+    }
+}
+```
+
+The custom processor must be added to the provider as below.
+
+```csharp
+services.AddOpenTelemetryTracerProvider(
+    (builder) => builder
+                .AddAspNetCoreInstrumentation()
+                .AddProcessor(new MyAspNetCoreEnrichingProcessor())
+                );
+```
+
 ## References
 
 * [Introduction to ASP.NET