feat: add o11y gRPC tracing

[shivanee-p]

Call TracerHelper within createApiCall() and update unit tests accordingly

• build StaticTraceContext from the internal telemetry information from CallSettings
• determine if calls are streaming or not
• add unit tests for fallback detection

[gemini-code-assist]

Code Review

This pull request integrates OpenTelemetry tracing into API calls within google-gax. It updates createApiCall to wrap invocations with traceAttempt when telemetry is enabled, captures static and dynamic trace contexts, and propagates the internal method name via constructSettings. Additionally, traceAttempt is refactored to synchronously handle both synchronous results and Promises. The feedback highlights two important issues: first, checking result instanceof Promise is fragile and fails to correctly track the lifetime of streaming calls (which return streams rather than Promises); second, the unit tests use a synchronous callback mock that masks this premature span termination. It is recommended to check for thenables and streams to properly manage span lifetimes, and to update the tests to invoke callbacks asynchronously.

[shivanee-p]

/gemini review

[shivanee-p]

Thanks for wiring traceCall, the callback wrapper, and getTracingBackstopMs through createApiCall, along with the new unit tests in apiCallable.ts. One small nit inline.

Ty for the comment! I updated the logic in the stack, but I removed the backstop parts. PR#9334 has the fix instead. The TLDR is that we never actually verified that REST calls were complete.