Fix HTTP tracing baggage propagation

[kexiongliu123]

This PR fixes a bug where tracing baggage was lost in Cinnamon when the YARPC tracing interceptor is enabled for HTTP. The root cause was that Cinnamon’s outbound middleware created a new span without inheriting baggage from the parent span. This broke end-to-end propagation, which was only caught by the Cinnamon integration test — currently the only E2E test covering baggage propagation with real HTTP transports. The fix manually copies baggage to the Cinnamon span.

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