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]

There is a data race on bg_tasks_ because bg_tasks_.push_back is called without holding the mu_ mutex. Since MaybeTriggerBackgroundFetch can be called concurrently from multiple threads, and CleanupCompletedTasks also modifies bg_tasks_ under mu_, this concurrent modification of std::vector leads to undefined behavior and potential crashes. Protect the push_back call with mu_ just like in AsyncConnectionTracing.

  std::unique_lock<std::mutex> lk(mu_);
  bg_tasks_.push_back(std::move(f));

[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 90.40853% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.18%. Comparing base (fd73bee) to head (9450ced).

Files with missing lines	Patch %	Lines
...cloud/storage/internal/async/connection_tracing.cc	35.00%	39 Missing ⚠️
.../cloud/storage/internal/tracing_connection_test.cc	95.12%	6 Missing ⚠️
...le/cloud/storage/internal/bucket_metadata_cache.cc	94.52%	4 Missing ⚠️
...oogle/cloud/storage/internal/tracing_connection.cc	98.31%	4 Missing ⚠️
...gle/cloud/storage/internal/bucket_metadata_cache.h	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16151      +/-   ##
==========================================
- Coverage   92.20%   92.18%   -0.02%     
==========================================
  Files        2264     2267       +3     
  Lines      208864   209376     +512     
==========================================
+ Hits       192579   193023     +444     
- Misses      16285    16353      +68     

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