WIP

src/Hosting/Hosting/src/Internal/HostingApplication.cs

@@ -129,7 +129,10 @@ public Activity? Activity
             {
                 if (HttpActivityFeature is null)
                 {
-                    HttpActivityFeature = new ActivityFeature(value!);
+                    if (value != null)
+                    {
+                        HttpActivityFeature = new ActivityFeature(value);
+                    }
                 }
                 else
                 {
@@ -144,7 +147,7 @@ public Activity? Activity
         public bool EventLogOrMetricsEnabled { get; set; }
 
         internal IHttpActivityFeature? HttpActivityFeature;
-        internal HttpMetricsTagsFeature? MetricsTagsFeature;
+        internal IHttpMetricsTagsFeature? MetricsTagsFeature;
 
         public void Reset()
         {

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

@@ -55,14 +55,14 @@ public void BeginRequest(HttpContext httpContext, HostingApplication.Context con
         if (_eventSource.IsEnabled() || _metrics.IsEnabled())
         {
             context.EventLogOrMetricsEnabled = true;
-            if (httpContext.Features.Get<IHttpMetricsTagsFeature>() is HttpMetricsTagsFeature feature)
+            if (httpContext.Features.Get<IHttpMetricsTagsFeature>() is IHttpMetricsTagsFeature feature)
             {
                 context.MetricsTagsFeature = feature;
             }
             else
             {
                 context.MetricsTagsFeature ??= new HttpMetricsTagsFeature();
-                httpContext.Features.Set<IHttpMetricsTagsFeature>(context.MetricsTagsFeature);
+                httpContext.Features.Set(context.MetricsTagsFeature);
             }
 
             startTimestamp = Stopwatch.GetTimestamp();
@@ -146,7 +146,7 @@ public void RequestEnd(HttpContext httpContext, Exception? exception, HostingApp
             if (context.EventLogOrMetricsEnabled)
             {
                 var route = httpContext.GetEndpoint()?.Metadata.GetMetadata<IRouteDiagnosticsMetadata>()?.Route;
-                var customTags = context.MetricsTagsFeature?.TagsList;
+                var customTags = context.MetricsTagsFeature?.Tags;
 
                 _metrics.RequestEnd(
                     httpContext.Request.Protocol,

src/Hosting/Hosting/src/Internal/HostingMetrics.cs

@@ -40,7 +40,7 @@ public void RequestStart(bool isHttps, string scheme, string method, HostString
         _currentRequestsCounter.Add(1, tags);
     }
 
-    public void RequestEnd(string protocol, bool isHttps, string scheme, string method, HostString host, string? route, int statusCode, Exception? exception, List<KeyValuePair<string, object?>>? customTags, long startTimestamp, long currentTimestamp)
+    public void RequestEnd(string protocol, bool isHttps, string scheme, string method, HostString host, string? route, int statusCode, Exception? exception, ICollection<KeyValuePair<string, object?>>? customTags, long startTimestamp, long currentTimestamp)
     {
         var tags = new TagList();
         InitializeRequestTags(ref tags, isHttps, scheme, method, host);
@@ -69,17 +69,33 @@ public void RequestEnd(string protocol, bool isHttps, string scheme, string meth
             }
             if (customTags != null)
             {
-                for (var i = 0; i < customTags.Count; i++)
-                {
-                    tags.Add(customTags[i]);
-                }
+                AddCustomTags(ref tags, customTags);
             }
 
             var duration = Stopwatch.GetElapsedTime(startTimestamp, currentTimestamp);
             _requestDuration.Record(duration.TotalSeconds, tags);
         }
     }
 
+    private static void AddCustomTags(ref TagList tags, ICollection<KeyValuePair<string, object?>> customTags)
+    {
+        // Skip allocating enumerator if tags collection is a list.
+        if (customTags is List<KeyValuePair<string, object?>> list)
+        {
+            for (var i = 0; i < list.Count; i++)
+            {
+                tags.Add(list[i]);
+            }
+        }
+        else
+        {
+            foreach (var tag in customTags)
+            {
+                tags.Add(tag);
+            }
+        }
+    }
+
     public void Dispose()
     {
         _meter.Dispose();

src/Hosting/Hosting/src/Internal/HttpMetricsTagsFeature.cs

@@ -7,7 +7,5 @@ namespace Microsoft.AspNetCore.Hosting;
 
 internal sealed class HttpMetricsTagsFeature : IHttpMetricsTagsFeature
 {
-    public ICollection<KeyValuePair<string, object?>> Tags => TagsList;
-
-    public List<KeyValuePair<string, object?>> TagsList { get; } = new List<KeyValuePair<string, object?>>();
+    public ICollection<KeyValuePair<string, object?>> Tags { get; } = new List<KeyValuePair<string, object?>>();
 }

src/Hosting/Hosting/test/HostingApplicationTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections;
+using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Diagnostics.Metrics;
 using Microsoft.AspNetCore.Hosting.Fakes;
@@ -108,6 +109,46 @@ static void AssertRequestDuration(Measurement<double> measurement, string protoc
         }
     }
 
+    [Fact]
+    public void MetricsFeatureUsedFromFeatureCollection()
+    {
+        // Arrange
+        var meterFactory = new TestMeterFactory();
+        var meterRegistry = new TestMeterRegistry(meterFactory.Meters);
+        var hostingApplication = CreateApplication(meterFactory: meterFactory);
+        var httpContext = new DefaultHttpContext();
+        var meter = meterFactory.Meters.Single();
+
+        using var requestDurationRecorder = new InstrumentRecorder<double>(meterRegistry, HostingMetrics.MeterName, "request-duration");
+        using var currentRequestsRecorder = new InstrumentRecorder<long>(meterRegistry, HostingMetrics.MeterName, "current-requests");
+
+        // Act/Assert
+        Assert.Equal(HostingMetrics.MeterName, meter.Name);
+        Assert.Null(meter.Version);
+
+        var feature = new TestHttpMetricsTagsFeature();
+        feature.Tags.Add(new KeyValuePair<string, object>("test", "Value!"));
+        httpContext.Features.Set<IHttpMetricsTagsFeature>(feature);
+        var context1 = hostingApplication.CreateContext(httpContext.Features);
+        context1.HttpContext.Response.StatusCode = StatusCodes.Status200OK;
+        hostingApplication.DisposeContext(context1, null);
+
+        Assert.Collection(currentRequestsRecorder.GetMeasurements(),
+            m => Assert.Equal(1, m.Value),
+            m => Assert.Equal(-1, m.Value));
+        Assert.Collection(requestDurationRecorder.GetMeasurements(),
+            m =>
+            {
+                Assert.True(m.Value > 0);
+                Assert.Equal("Value!", (string)m.Tags.ToArray().Single(t => t.Key == "test").Value);
+            });
+    }
+
+    private class TestHttpMetricsTagsFeature : IHttpMetricsTagsFeature
+    {
+        public ICollection<KeyValuePair<string, object>> Tags { get; } = new Collection<KeyValuePair<string, object>>();
+    }
+
     [Fact]
     public void DisposeContextDoesNotClearHttpContextIfDefaultHttpContextFactoryUsed()
     {
@@ -204,7 +245,9 @@ public void IHttpActivityFeatureIsPopulated()
         var initialActivity = Activity.Current;
 
         // Create nested dummy Activity
-        using var _ = dummySource.StartActivity("DummyActivity");
+        using var dummyActivity = dummySource.StartActivity("DummyActivity");
+        Assert.NotNull(dummyActivity);
+        Assert.Equal(Activity.Current, dummyActivity);
 
         Assert.Same(initialActivity, activityFeature.Activity);
         Assert.Null(activityFeature.Activity.ParentId);
@@ -247,7 +290,9 @@ public void IHttpActivityFeatureIsAssignedToIfItExists()
         var initialActivity = Activity.Current;
 
         // Create nested dummy Activity
-        using var _ = dummySource.StartActivity("DummyActivity");
+        using var dummyActivity = dummySource.StartActivity("DummyActivity");
+        Assert.NotNull(dummyActivity);
+        Assert.Equal(Activity.Current, dummyActivity);
 
         Assert.Same(initialActivity, activityFeature.Activity);
         Assert.Null(activityFeature.Activity.ParentId);

src/Servers/Kestrel/Core/src/Internal/Infrastructure/KestrelConnectionOfT.cs

@@ -40,15 +40,22 @@ internal async Task ExecuteAsync()
         var connectionContext = _transportConnection;
         var metricsConnectionDurationEnabled = _serviceContext.Metrics.IsConnectionDurationEnabled();
         var startTimestamp = 0L;
-        ConnectionMetricsTagsFeature? metricsTagsFeature = null;
+        IConnectionMetricsTagsFeature? metricsTagsFeature = null;
         Exception? unhandledException = null;
 
         if (metricsConnectionDurationEnabled)
         {
-            startTimestamp = Stopwatch.GetTimestamp();
+            if (connectionContext.Features.Get<IConnectionMetricsTagsFeature>() is IConnectionMetricsTagsFeature feature)
+            {
+                metricsTagsFeature = feature;
+            }
+            else
+            {
+                metricsTagsFeature = new ConnectionMetricsTagsFeature();
+                connectionContext.Features.Set(metricsTagsFeature);
+            }
 
-            metricsTagsFeature = new ConnectionMetricsTagsFeature();
-            connectionContext.Features.Set<IConnectionMetricsTagsFeature>(metricsTagsFeature);
+            startTimestamp = Stopwatch.GetTimestamp();
         }
 
         try
@@ -85,7 +92,7 @@ internal async Task ExecuteAsync()
 
             Logger.ConnectionStop(connectionContext.ConnectionId);
             KestrelEventSource.Log.ConnectionStop(connectionContext);
-            _serviceContext.Metrics.ConnectionStop(connectionContext, unhandledException, metricsTagsFeature?.TagsList, startTimestamp, currentTimestamp);
+            _serviceContext.Metrics.ConnectionStop(connectionContext, unhandledException, metricsTagsFeature?.Tags, startTimestamp, currentTimestamp);
 
             // Dispose the transport connection, this needs to happen before removing it from the
             // connection manager so that we only signal completion of this connection after the transport
@@ -98,7 +105,6 @@ internal async Task ExecuteAsync()
 
     private sealed class ConnectionMetricsTagsFeature : IConnectionMetricsTagsFeature
     {
-        public ICollection<KeyValuePair<string, object?>> Tags => TagsList;
-        public List<KeyValuePair<string, object?>> TagsList { get; } = new List<KeyValuePair<string, object?>>();
+        public ICollection<KeyValuePair<string, object?>> Tags { get; } = new List<KeyValuePair<string, object?>>();
     }
 }

src/Servers/Kestrel/Core/src/Internal/Infrastructure/KestrelMetrics.cs

@@ -80,7 +80,7 @@ private void ConnectionStartCore(BaseConnectionContext connection)
         _currentConnectionsCounter.Add(1, tags);
     }
 
-    public void ConnectionStop(BaseConnectionContext connection, Exception? exception, List<KeyValuePair<string, object?>>? customTags, long startTimestamp, long currentTimestamp)
+    public void ConnectionStop(BaseConnectionContext connection, Exception? exception, ICollection<KeyValuePair<string, object?>>? customTags, long startTimestamp, long currentTimestamp)
     {
         if (_currentConnectionsCounter.Enabled || _connectionDuration.Enabled)
         {
@@ -89,7 +89,7 @@ public void ConnectionStop(BaseConnectionContext connection, Exception? exceptio
     }
 
     [MethodImpl(MethodImplOptions.NoInlining)]
-    private void ConnectionStopCore(BaseConnectionContext connection, Exception? exception, List<KeyValuePair<string, object?>>? customTags, long startTimestamp, long currentTimestamp)
+    private void ConnectionStopCore(BaseConnectionContext connection, Exception? exception, ICollection<KeyValuePair<string, object?>>? customTags, long startTimestamp, long currentTimestamp)
     {
         var tags = new TagList();
         InitializeConnectionTags(ref tags, connection);
@@ -105,16 +105,32 @@ private void ConnectionStopCore(BaseConnectionContext connection, Exception? exc
         // Add custom tags for duration.
         if (customTags != null)
         {
-            for (var i = 0; i < customTags.Count; i++)
-            {
-                tags.Add(customTags[i]);
-            }
+            AddCustomTags(ref tags, customTags);
         }
 
         var duration = Stopwatch.GetElapsedTime(startTimestamp, currentTimestamp);
         _connectionDuration.Record(duration.TotalSeconds, tags);
     }
 
+    private static void AddCustomTags(ref TagList tags, ICollection<KeyValuePair<string, object?>> customTags)
+    {
+        // Skip allocating enumerator if tags collection is a list.
+        if (customTags is List<KeyValuePair<string, object?>> list)
+        {
+            for (var i = 0; i < list.Count; i++)
+            {
+                tags.Add(list[i]);
+            }
+        }
+        else
+        {
+            foreach (var tag in customTags)
+            {
+                tags.Add(tag);
+            }
+        }
+    }
+
     public void ConnectionRejected(BaseConnectionContext connection)
     {
         if (_rejectedConnectionsCounter.Enabled)