spanner request id prototype

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a generalized OperationContext mechanism to inject request IDs and track attempt lifecycles across retries, generalizing the Bigtable generator option and implementing a robust request ID tracking system in the Spanner client. The review feedback highlights critical thread-safety and correctness issues: a broken double-checked locking pattern in DefaultSpannerOperationContextFactory::StaticPrefix() that introduces data races, a thread-safety hazard in ProcessRandomId() due to returning a reference to a mutable static string instead of returning by value, and a bug in BatchWriteGuard where premature stream cancellation incorrectly reports a successful status.

[codecov]

Codecov Report

❌ Patch coverage is 85.80470% with 157 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (6e0a08e) to head (28e6fe0).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/cloud/spanner/internal/spanner_auth_decorator.cc	2.70%	36 Missing ⚠️
...gle/cloud/spanner/internal/spanner_tracing_stub.cc	5.40%	35 Missing ⚠️
google/cloud/spanner/internal/session_pool.cc	81.81%	20 Missing ⚠️
...nner/internal/spanner_operation_context_factory.cc	79.72%	15 Missing ⚠️
...loud/spanner/internal/spanner_logging_decorator.cc	76.92%	12 Missing ⚠️
...internal/spanner_operation_context_factory_test.cc	85.89%	11 Missing ⚠️
.../cloud/spanner/internal/spanner_request_id_test.cc	86.20%	8 Missing ⚠️
generator/internal/descriptor_utils.cc	22.22%	7 Missing ⚠️
...oud/spanner/internal/spanner_metadata_decorator.cc	75.00%	6 Missing ⚠️
google/cloud/spanner/internal/connection_impl.cc	96.35%	5 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16448      +/-   ##
==========================================
- Coverage   92.28%   92.24%   -0.05%     
==========================================
  Files        2246     2254       +8     
  Lines      212894   213541     +647     
==========================================
+ Hits       196474   196973     +499     
- Misses      16420    16568     +148     

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