impl(o11y): add operation name support to Poller interface

[haphungw]

Allow tracing decorators to dynamically query LRO operation IDs from underlying poller for telemetry recording.

Because PollerImpl is encapsulated as a generic inside the telemetry decorator, we can't do direct field access (like self.inner.operation) on the generic type. Declaring this standard method on the Poller trait supports LRO tracing attribute propagation without contaminating public structs.

[gemini-code-assist]

Code Review

This pull request introduces an operation_name method to the Poller trait and its various implementations to allow retrieving the name of a long-running operation. The reviewer suggests changing the return type from Option<String> to Option<&str> to avoid unnecessary allocations and clones, making the API more idiomatic and efficient. Additionally, feedback was provided to use as_deref() in implementations and to update test assertions to use string literals for better independence.

[codecov]

Codecov Report

❌ Patch coverage is 60.46512% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.88%. Comparing base (6e7650c) to head (201e963).

Files with missing lines	Patch %	Lines
src/lro/src/internal/aip151.rs	45.45%	6 Missing ⚠️
src/lro/src/internal/discovery.rs	77.77%	6 Missing ⚠️
src/lro/src/internal/either.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5694      +/-   ##
==========================================
- Coverage   97.91%   97.88%   -0.03%     
==========================================
  Files         226      226              
  Lines       55347    55390      +43     
==========================================
+ Hits        54191    54219      +28     
- Misses       1156     1171      +15     

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[coryan]

This is fine. It is a new public API, but not a big deal. But I just realized that backoff() is also public, and I wonder if we should have not made it so...

[haphungw]

I found an alternative to avoid touching the public API: #5695.

Instead of adding operation_name() to query the inner poller, Tracing<P>::poll now enters the T2 span scope before delegating the poll call. The T2 span is now active in the thread context, and the pollers can record the operation name directly onto it using tracing::Span::current().record().

If you think this works, I can go ahead and close this PR.