[HttpClient and ASP.NET Core] Add note for relying on activity duration for metric data (#4183)

* add note on activity duration

* Update src/OpenTelemetry.Instrumentation.AspNetCore/Implementation/HttpInMetricsListener.cs

Co-authored-by: Reiley Yang <reyang@microsoft.com>

* add todo

* re

---------

Co-authored-by: Reiley Yang <reyang@microsoft.com>

src/OpenTelemetry.Instrumentation.AspNetCore/Implementation/HttpInMetricsListener.cs

@@ -110,6 +110,9 @@ namespace OpenTelemetry.Instrumentation.AspNetCore.Implementation
                     }
                 }
 
+                // We are relying here on ASP.NET Core to set duration before writing the stop event.
+                // https://github.com/dotnet/aspnetcore/blob/d6fa351048617ae1c8b47493ba1abbe94c3a24cf/src/Hosting/Hosting/src/Internal/HostingApplicationDiagnostics.cs#L449
+                // TODO: Follow up with .NET team if we can continue to rely on this behavior.
                 this.httpServerDuration.Record(Activity.Current.Duration.TotalMilliseconds, tags);
             }
         }

src/OpenTelemetry.Instrumentation.Http/Implementation/HttpHandlerMetricsDiagnosticListener.cs

@@ -62,6 +62,9 @@ namespace OpenTelemetry.Instrumentation.Http.Implementation
                         tags.Add(new KeyValuePair<string, object>(SemanticConventions.AttributeNetPeerPort, request.RequestUri.Port));
                     }
 
+                    // We are relying here on HttpClient library to set duration before writing the stop event.
+                    // https://github.com/dotnet/runtime/blob/90603686d314147017c8bbe1fa8965776ce607d0/src/libraries/System.Net.Http/src/System/Net/Http/DiagnosticsHandler.cs#L178
+                    // TODO: Follow up with .NET team if we can continue to rely on this behavior.
                     this.httpClientDuration.Record(activity.Duration.TotalMilliseconds, tags);
                 }
             }