Reduce allocations when Activity is enabled (#11020)

- Remove string allocations caused by DiagnosticSource.Stop/StartActivity
- Pass the HttpContext directly as the object for StartActivity and StopActivity to avoid the anonymous object allocation.
- Though it's a bit ugly, added an HttpContext property to DefaultHttpContext to avoid breaking back-compat (which had to do reflection to get the HttpContext property anyways)

src/Hosting/Hosting/src/Internal/HostingApplicationDiagnostics.cs

@@ -16,7 +16,8 @@ namespace Microsoft.AspNetCore.Hosting.Internal
         private static readonly double TimestampToTicks = TimeSpan.TicksPerSecond / (double)Stopwatch.Frequency;
 
         private const string ActivityName = "Microsoft.AspNetCore.Hosting.HttpRequestIn";
-        private const string ActivityStartKey = "Microsoft.AspNetCore.Hosting.HttpRequestIn.Start";
+        private const string ActivityStartKey = ActivityName + ".Start";
+        private const string ActivityStopKey = ActivityName + ".Stop";
 
         private const string DeprecatedDiagnosticsBeginRequestKey = "Microsoft.AspNetCore.Hosting.BeginRequest";
         private const string DeprecatedDiagnosticsEndRequestKey = "Microsoft.AspNetCore.Hosting.EndRequest";
@@ -278,7 +279,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             if (_diagnosticListener.IsEnabled(ActivityStartKey))
             {
                 hasDiagnosticListener = true;
-                _diagnosticListener.StartActivity(activity, new { HttpContext = httpContext });
+                StartActivity(activity, httpContext);
             }
             else
             {
@@ -293,12 +294,32 @@ namespace Microsoft.AspNetCore.Hosting.Internal
         {
             if (hasDiagnosticListener)
             {
-                _diagnosticListener.StopActivity(activity, new { HttpContext = httpContext });
+                StopActivity(activity, httpContext);
             }
             else
             {
                 activity.Stop();
             }
         }
+
+        // These are versions of DiagnosticSource.Start/StopActivity that don't allocate strings per call (see https://github.com/dotnet/corefx/issues/37055)
+        private Activity StartActivity(Activity activity, HttpContext httpContext)
+        {
+            activity.Start();
+            _diagnosticListener.Write(ActivityStartKey, httpContext);
+            return activity;
+        }
+
+        private void StopActivity(Activity activity, HttpContext httpContext)
+        {
+            // Stop sets the end time if it was unset, but we want it set before we issue the write
+            // so we do it now.   
+            if (activity.Duration == TimeSpan.Zero)
+            {
+                activity.SetEndTime(DateTime.UtcNow);
+            }
+            _diagnosticListener.Write(ActivityStopKey, httpContext);
+            activity.Stop();    // Resets Activity.Current (we want this after the Write)
+        }
     }
 }

src/Http/Http/ref/Microsoft.AspNetCore.Http.netcoreapp3.0.cs

@@ -24,6 +24,8 @@ namespace Microsoft.AspNetCore.Http
         public override Microsoft.AspNetCore.Http.ConnectionInfo Connection { get { throw null; } }
         public override Microsoft.AspNetCore.Http.Features.IFeatureCollection Features { get { throw null; } }
         public Microsoft.AspNetCore.Http.Features.FormOptions FormOptions { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+        [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
+        public Microsoft.AspNetCore.Http.HttpContext HttpContext { get { throw null; } }
         public override System.Collections.Generic.IDictionary<object, object> Items { get { throw null; } set { } }
         public override Microsoft.AspNetCore.Http.HttpRequest Request { get { throw null; } }
         public override System.Threading.CancellationToken RequestAborted { get { throw null; } set { } }

src/Http/Http/src/DefaultHttpContext.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Security.Claims;
 using System.Threading;
 using Microsoft.AspNetCore.Http.Features;
@@ -157,6 +158,12 @@ namespace Microsoft.AspNetCore.Http
             }
         }
 
+        // This property exists because of backwards compatibility.
+        // We send an anonymous object with an HttpContext property
+        // via DiagnosticSource in various events throughout the pipeline. Instead
+        // we just send the HttpContext to avoid extra allocations
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        public HttpContext HttpContext => this;
 
         public override void Abort()
         {