Add docs about new SignalR tracing in .NET 9

[JamesNK]

Description

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

• Add separate activities for Hub methods aspnetcore#55439
• Propagate trace parent to SignalR hub invocations aspnetcore#57049
• Add client span to SignalR .NET client aspnetcore#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

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Associated WorkItem - 362956

[BrennanConroy]

We should consider following the same format as gRPC.

SignalR has two activity names, one for the server, one for the client
dotnet/aspnetcore#51557 (comment)