Listen to SignalR server activities by default in ASP.NET Core

[BrennanConroy]

Fixes #1998
Design discussion issue #1998

Changes

Adds activity source "Microsoft.AspNetCore.SignalR.Server" when calling AddAspNetCoreInstrumentation.
This activity source is new in .NET 9.0.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[cijothomas]

are these long (as in, really long) running server spans? I recall ApplicationInsights users complained about recording such long running spans, as that'd skew the latency calculations, and ended up ignoring those spans by default...

dotnet/aspnetcore#32084

[cijothomas]

If this is the case, then we should not do this by default, as there is no API to "remove" a source once added to the provider....

[BrennanConroy]

The only long running span should be the http one that has the long running tag. All of the newly added SignalR spans are only for a specific operation.

Most of the spans added by SignalR add the long running span as a link, but individually they are short lived operations.

The OnConnectedAsync and OnDisconnectedAsync spans are the only 2 that are parented under the long running span, but they are both very short lived spans themselves.

[rajkumar-rangaraj]

A few years back, I collaborated with your team to introduce the http.long_running attribute in SignalR to help filter long-running HTTP requests in Application Insights .NET SDK. This effort was driven by significant customer feedback about long-running SignalR requests distorting request duration metrics.

With http.long_running part of the default ASP.NET Core instrumentation, customers might face similar issues if long-running spans are automatically included without a suppression mechanism. I recommend considering an option to filter these spans by default or providing a toggle to enable them explicitly before rolling this out.