Expose TraceIdentifier on Httpcontext

Author: benaadams

src/Microsoft.AspNet.Http.Abstractions/HttpContext.cs

@@ -36,6 +36,8 @@ public abstract class HttpContext : IDisposable
 
         public abstract ISession Session { get; set; }
 
+        public abstract string TraceIdentifier { get; set; }
+
         public abstract void Abort();
 
         public abstract void Dispose();

src/Microsoft.AspNet.Http/DefaultHttpContext.cs

@@ -180,6 +180,12 @@ public override CancellationToken RequestAborted
             set { LifetimeFeature.RequestAborted = value; }
         }
 
+        public override string TraceIdentifier
+        {
+            get { return RequestIdentifierFeature.TraceIdentifier; }
+            set { RequestIdentifierFeature.TraceIdentifier = value; }
+        }
+
         public override ISession Session
         {
             get