[HttpClient & ASP.NET Core] Document request durations (#5079)

src/OpenTelemetry.Instrumentation.AspNetCore/README.md

@@ -235,6 +235,23 @@ This instrumentation automatically sets Activity Status to Error if an unhandled
 exception is thrown. Additionally, `RecordException` feature may be turned on,
 to store the exception to the Activity itself as ActivityEvent.
 
+## Activity Duration and http.server.request.duration metric calculation
+
+`Activity.Duration` and `http.server.request.duration` values represents the
+time used to handle an inbound HTTP request as measured at the hosting layer of
+ASP.NET Core. The time measurement starts once the underlying web host has:
+
+* Sufficiently parsed the HTTP request headers on the inbound network stream to
+  identify the new request.
+* Initialized the context data structures such as the
+  [HttpContext](https://learn.microsoft.com/dotnet/api/microsoft.aspnetcore.http.httpcontext).
+
+The time ends when:
+
+* The ASP.NET Core handler pipeline is finished executing.
+* All response data has been sent.
+* The context data structures for the request are being disposed.
+
 ## Troubleshooting
 
 This component uses an

src/OpenTelemetry.Instrumentation.Http/README.md

@@ -312,6 +312,13 @@ This instrumentation automatically sets Activity Status to Error if the Http
 StatusCode is >= 400. Additionally, `RecordException` feature may be turned on,
 to store the exception to the Activity itself as ActivityEvent.
 
+## Activity Duration and http.client.request.duration metric calculation
+
+`Activity.Duration` and `http.client.request.duration` values represents the
+time the underlying client handler takes to complete the request. Completing the
+request includes the time up to reading response headers from the network
+stream. It doesn't include the time spent reading the response body.
+
 ## Troubleshooting
 
 This component uses an