feat(o11y): track destination_id state in LroRecorder

[haphungw]

Introduce a synchronized destination_id state to google_cloud_lro::LroRecorder to persist the extracted resource destination ID across polling iterations.

This serves as the underlying plumbing for generated tracing decorators to inject gcp.resource.destination.id into LRO spans.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.89%. Comparing base (1059b90) to head (8af79f6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5819   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files         226      226           
  Lines       57713    57713           
=======================================
+ Hits        56496    56497    +1     
+ Misses       1217     1216    -1     

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[gemini-code-assist]

Code Review

This pull request refactors the LRO tracing implementation by removing the POLL_ATTEMPT_COUNT task-local variable and consolidating the active LRO span and polling attempt count into an immutable, thread-safe LroRecorder struct. It also cleans up conditional compilation attributes (#[cfg(google_cloud_unstable_tracing)]) across several files. Feedback on the changes suggests simplifying the resource_destination_id method in src/lro/src/internal/tracing.rs using the ? operator on the Option returned by ok() to make the code more concise and idiomatic.