HostingMetrics: empty "http.route" tags should be set to "/"

These changes have been decided in #62431

Author: Elanis

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

@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Metrics;
+
 using Microsoft.AspNetCore.Http;
 
 namespace Microsoft.AspNetCore.Hosting;
@@ -66,10 +67,12 @@ public void RequestEnd(string protocol, string scheme, string method, string? ro
 
             // Add information gathered during request.
             tags.Add("http.response.status_code", GetBoxedStatusCode(statusCode));
-            if (!string.IsNullOrEmpty(route))
+            var httpRoute = route;
+            if (string.IsNullOrEmpty(httpRoute))
             {
-                tags.Add("http.route", route);
+                httpRoute = "/";
             }
+            tags.Add("http.route", httpRoute);
 
             // Add before some built in tags so custom tags are prioritized when dealing with duplicates.
             if (customTags != null)

src/Hosting/Hosting/test/HostingApplicationDiagnosticsTests.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics.Metrics;
 using System.Diagnostics.Tracing;
 using System.Reflection;
+
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
@@ -15,6 +16,7 @@
 using Microsoft.Extensions.Diagnostics.Metrics.Testing;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Testing;
+
 using Moq;
 
 namespace Microsoft.AspNetCore.Hosting.Tests;
@@ -358,7 +360,7 @@ private sealed class EmptyRouteDiagnosticsMetadata : IRouteDiagnosticsMetadata
     }
 
     [Fact]
-    public void Metrics_Route_RouteTagMissingWhenEmpty()
+    public void Metrics_Route_RouteTagIsRootWhenEmpty()
     {
         // Arrange
         var hostingEventSource = new HostingEventSource(Guid.NewGuid().ToString());
@@ -413,7 +415,7 @@ public void Metrics_Route_RouteTagMissingWhenEmpty()
             m =>
             {
                 Assert.True(m.Value > 0);
-                Assert.False(m.Tags.ContainsKey("http.route"));
+                Assert.Equal("/", m.Tags["http.route"]);
             });
     }