.NET Core 3: Add Server-Timing trailer

This adds an option (default off) to enable Server-Timing headers for requests. It's a trailer here so requires Kestrel/feature support. If this works well, we can look at adding a netcoreapp2.2 build for MiniProfiler.AspNetCore* which should also support the headers.

I think this works, but it's hard to verify because of an https:// issue in dotnet/aspnetcore#8952 which breaks Chrome and Firefox that won't be resolved until preview 7. Edge can connect (due to never fixing their TLS leniency...), but not show the trailers. Figures :)

samples/Samples.AspNetCore3/Startup.cs

@@ -76,6 +76,9 @@ public void ConfigureServices(IServiceCollection services)
                 // Optionally disable "Connection Open()", "Connection Close()" (and async variants).
                 //options.TrackConnectionOpenClose = false;
 
+                // Enabled sending the Server-Timing header on responses
+                options.EnableServerTimingHeader = true;
+
                 options.IgnoredPaths.Add("/lib");
                 options.IgnoredPaths.Add("/css");
                 options.IgnoredPaths.Add("/js");

src/MiniProfiler.AspNetCore/MiniProfilerMiddleware.cs

@@ -89,6 +89,16 @@ public async Task Invoke(HttpContext context)
                 {
                     await SetHeadersAndState(context, mp).ConfigureAwait(false);
                 }
+
+#if NETCOREAPP3_0
+                var appendServerTimingHeader = Options.EnableServerTimingHeader && context.Response.SupportsTrailers();
+                if (appendServerTimingHeader)
+                {
+                    context.Response.DeclareTrailer("Server-Timing");
+                    appendServerTimingHeader = true;
+                }
+#endif
+
                 // Execute the pipe
 #pragma warning disable RCS1090 // Call 'ConfigureAwait(false)'.
                 await _next(context);
@@ -97,6 +107,13 @@ public async Task Invoke(HttpContext context)
                 EnsureName(mp, context);
                 // Stop (and record)
                 await mp.StopAsync().ConfigureAwait(false);
+
+#if NETCOREAPP3_0 // TODO: Evaluate if this works after http/2 local support in preview 7, maybe backport to netcoreapp2.2
+                if (appendServerTimingHeader && mp != null)
+                {
+                    context.Response.AppendTrailer("Server-Timing", mp.GetServerTimingHeader());
+                }
+#endif
             }
             else
             {

src/MiniProfiler.AspNetCore/MiniProfilerOptions.cs

@@ -40,5 +40,12 @@ public class MiniProfilerOptions : MiniProfilerBaseOptions
         /// Function to provide the unique user ID based on the request, to store MiniProfiler IDs user
         /// </summary>
         public Func<HttpRequest, string> UserIdProvider { get; set; } = request => request.HttpContext.Connection.RemoteIpAddress?.ToString();
+
+#if NETCOREAPP3_0
+        /// <summary>
+        /// Whether to add a Server-Timing header after profiling a request. Only supported in .NET Core 3.0 and higher.
+        /// </summary>
+        public bool EnableServerTimingHeader { get; set; }
+#endif
     }
 }