WN: SignalR distributed tracing: Pre-work moving as is to include (#33547)

* WN: SingalR distributed tracing

* Prep only PR so we have something to diff against on new PR that contains the edits.

* Combined hub and client ActivitiesSource in one new file.

aspnetcore/release-notes/aspnetcore-9/includes/signalr-distributed-tracing-improvements.md

@@ -0,0 +1,58 @@
+### Improved Activities for SignalR
+
+SignalR now has an ActivitySource for both the hub server and client.
+
+#### .NET SignalR server ActivitySource
+
+The SignalR ActivitySource named `Microsoft.AspNetCore.SignalR.Server` emits events for hub method calls:
+
+* Every method is its own activity, so anything that emits an activity during the hub method call is under the hub method activity.
+* Hub method activities don't have a parent. This means they are not bundled under the long-running SignalR connection.
+
+The following example uses the [.NET Aspire dashboard](/dotnet/aspire/fundamentals/dashboard/overview?tabs=bash#using-the-dashboard-with-net-aspire-projects) and the [OpenTelemetry](https://www.nuget.org/packages/OpenTelemetry.Extensions.Hosting) packages:
+
+```xml
+<PackageReference Include="OpenTelemetry.Exporter.OpenTelemetryProtocol" Version="1.9.0" />
+<PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.9.0" />
+<PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="1.9.0" />
+```
+
+Add the following startup code to the `Program.cs` file:
+
+```csharp
+// Set OTEL_EXPORTER_OTLP_ENDPOINT environment variable depending on where your OTEL endpoint is
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Services.AddRazorPages();
+builder.Services.AddSignalR();
+
+builder.Services.AddOpenTelemetry()
+    .WithTracing(tracing =>
+    {
+        if (builder.Environment.IsDevelopment())
+        {
+            // We want to view all traces in development
+            tracing.SetSampler(new AlwaysOnSampler());
+        }
+
+        tracing.AddAspNetCoreInstrumentation();
+        tracing.AddSource("Microsoft.AspNetCore.SignalR.Server");
+    });
+
+builder.Services.ConfigureOpenTelemetryTracerProvider(tracing => tracing.AddOtlpExporter());
+```
+
+The following is example output from the Aspire Dashboard:
+
+:::image type="content" source="~/release-notes/aspnetcore-9/_static/signalr-activites-events.png" alt-text="Activity list for SignalR Hub method call events":::
+
+#### .NET SignalR client ActivitySource
+
+The SignalR ActivitySource named "Microsoft.AspNetCore.SignalR.Client" emits events for a SignalR client:
+
+* .NET SignalR client has an `ActivitySource` named "Microsoft.AspNetCore.SignalR.Client". Hub invocations now create a client span. Note that other SignalR clients, such as the JavaScript client, don't support tracing. This feature will be added to more clients in future releases.
+* Hub invocations on the client and server support [context propagation](https://opentelemetry.io/docs/concepts/context-propagation/). Propagating the trace context enables true distributed tracing. It's now possible to see invocations flow from the client to the server and back.
+
+Here's how these new activities look in the [.NET Aspire dashboard](https://learn.microsoft.com/dotnet/aspire/fundamentals/dashboard/overview?tabs=bash#standalone-mode):
+
+![SignalR distributed tracing in Aspire dashboard](~/release-notes/aspnetcore-9/_static/signalr-distributed-tracing-aspire-dashboard.png)