feat(storage): add resource span attributes for ACO ( App Centric Observability ) for async client

[bajajneha27]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a BucketMetadataCache to cache bucket metadata (ID and location) and uses it to enrich OpenTelemetry spans with destination resource attributes in both TracingConnection and AsyncConnectionTracing. When metadata is not cached, a background fetch is triggered. The review feedback highlights several important issues: a critical data race on bg_tasks_ in TracingConnection::MaybeTriggerBackgroundFetch due to a lack of mutex protection during push_back, potential undefined behavior in BucketMetadataCache::Put if max_size is initialized to 0, unnormalized bucket names on permission denied errors in TracingConnection, and performance concerns regarding the expensive creation of a new StorageConnection on every background fetch in AsyncConnectionTracing.

[gemini-code-assist]

To be consistent with AsyncConnectionTracing and to handle any potentially unnormalized bucket names safely, normalize the bucket_name before constructing the fallback resource ID on kPermissionDenied errors.

    } else if (result.status().code() == StatusCode::kPermissionDenied) {
      auto const normalized = BucketMetadataCache::NormalizeBucketName(bucket_name);
      cache().Put(bucket_name, {"projects/_/buckets/" + normalized, "global"});
    }

[codecov]

Codecov Report

❌ Patch coverage is 96.01722% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.29%. Comparing base (6fd3745) to head (4244a97).

Files with missing lines	Patch %	Lines
.../cloud/storage/internal/tracing_connection_test.cc	95.73%	9 Missing ⚠️
...cloud/storage/internal/async/connection_tracing.cc	93.80%	7 Missing ⚠️
...gle/cloud/storage/internal/storage_tracing_stub.cc	0.00%	7 Missing ⚠️
google/cloud/storage/grpc_plugin.cc	57.14%	3 Missing ⚠️
.../storage/internal/storage_round_robin_decorator.cc	0.00%	3 Missing ⚠️
google/cloud/storage/async/idempotency_policy.cc	50.00%	2 Missing ⚠️
...le/cloud/storage/internal/bucket_metadata_cache.cc	96.87%	2 Missing ⚠️
...oogle/cloud/storage/internal/tracing_connection.cc	99.10%	2 Missing ⚠️
google/cloud/storage/async/client_test.cc	98.14%	1 Missing ⚠️
google/cloud/storage/internal/grpc/stub.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16151      +/-   ##
==========================================
+ Coverage   92.27%   92.29%   +0.01%     
==========================================
  Files        2214     2217       +3     
  Lines      206537   207397     +860     
==========================================
+ Hits       190589   191408     +819     
- Misses      15948    15989      +41     

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