Copy service.name baggage onto all span attributes

[nikhilNava]

Summary

The service.name baggage value, set via BaggageBuilder.OperationSource(...), was never copied onto span attributes because ServiceNameKey was missing from ActivityProcessor's AttributeKeys allow-list. As a result, OperationSource(...) had no effect on emitted telemetry.

Change

Added OpenTelemetryConstants.ServiceNameKey to the global AttributeKeys list in ActivityProcessor. CoalesceTag only applies the tag when the baggage value is non-empty, so it is copied to every span only when set.

Audit

Reviewed all baggage keys written by BaggageBuilder and TurnContextExtensions against the processor allow-lists. service.name was the only baggage attribute set but not copied to spans; all others were already covered (server.address/server.port remain scoped to invoke_agent spans intentionally).

Testing

• dotnet build succeeds
• All 331 Observability.Runtime tests pass

[github-actions]

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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[Copilot]

Pull request overview

This PR ensures that the service.name value stored in OpenTelemetry baggage (via BaggageBuilder.OperationSource(...)) is propagated onto span attributes by adding OpenTelemetryConstants.ServiceNameKey to ActivityProcessor’s global AttributeKeys allow-list.

Changes:

• Added OpenTelemetryConstants.ServiceNameKey to the ActivityProcessor attribute allow-list so it is coalesced onto every started span when present.