Add docs about new SignalR tracing in .NET 9

[JamesNK]

Description

Distributed tracing was added for SignalR Core in client and server:

• https://github.com/dotnet/aspnetcore/pull/55439
• https://github.com/dotnet/aspnetcore/pull/57049
• https://github.com/dotnet/aspnetcore/pull/57101

Should be added to docs for .NET 9:

cc @BrennanConroy

Page URL

https://learn.microsoft.com/en-us/aspnet/core/signalr/diagnostics?view=aspnetcore-8.0

Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/signalr/diagnostics.md

Document ID

3e302985-8504-d333-3222-ab8da8902da2

Article author

@bradygaster

[BrennanConroy]

We should consider following the same format as gRPC.

SignalR has two activity names, one for the server, one for the client https://github.com/dotnet/aspnetcore/issues/51557#issuecomment-2136051137