test(o11y): add Discovery LRO tracing success integration test

[haphungw]

Introduce tracing tests for success cases for Discovery-based client pollers.

Stacked on top of https://github.com/googleapis/google-cloud-rust/pull/5889.



[gemini-code-assist]

Code Review

This pull request introduces integration tests for Long-Running Operation (LRO) tracing within the o11y integration tests, covering successful polling, logical errors, transient and permanent RPC errors, and discovery-based LRO success. The review feedback suggests removing a redundant #[cfg(google_cloud_unstable_tracing)] attribute within tests/o11y/src/lro_tracing.rs since the entire file is already gated by this configuration at the module level.

[gemini-code-assist]

The #[cfg(google_cloud_unstable_tracing)] attribute is redundant here because the entire file is already gated by #![cfg(google_cloud_unstable_tracing)] at the module level (line 15). Removing it simplifies the code and improves readability.

Suggested change

	#[cfg(google_cloud_unstable_tracing)]
	google_cloud_lro::record_polling_attributes!(&span);
	google_cloud_lro::record_polling_attributes!(&span);

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.89%. Comparing base (d059878) to head (4e56084).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5891   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files         233      233           
  Lines       59202    59202           
=======================================
  Hits        57958    57958           
  Misses       1244     1244           

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