feat(storage): add feature header for multi stream feature

[v-pratap]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a FeatureTracker class to track and report features adopted during storage operations, specifically tracking multi-stream usage in multi-range downloads (kMultiStreamInMRD). The tracked features are encoded as a bitmask, Base64-encoded, and sent via the x-goog-storage-cpp-features metadata header. Feedback was provided on FeatureTracker::RegisterFeature to defensively check that the feature enum value is less than 32 before shifting, preventing potential undefined behavior if more features are added in the future.

[gemini-code-assist]

Shifting 1U by static_cast<std::uint32_t>(feature) can result in undefined behavior if feature has a value of 32 or greater (since mask_ is a 32-bit atomic integer). To prevent potential undefined behavior as new features are added to the TrackedFeature enum in the future, we should add a safety check to ensure the shift value is strictly less than 32.

  void RegisterFeature(TrackedFeature feature) {
    auto const shift = static_cast<std::uint32_t>(feature);
    if (shift < 32) {
      mask_.fetch_or(1U << shift, std::memory_order_relaxed);
    }
  }

References

1. Prefer defensive code to prevent potential undefined behavior or contract changes in the future.

[codecov]

Codecov Report

❌ Patch coverage is 95.71429% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.20%. Comparing base (cc530e2) to head (d0a921e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rage/internal/async/object_descriptor_impl_test.cc	93.02%	3 Missing ⚠️
google/cloud/storage/internal/async/open_object.cc	80.00%	2 Missing ⚠️
...d/storage/internal/grpc/configure_client_context.h	90.90%	1 Missing ⚠️
google/cloud/storage/internal/rest/stub.cc	88.88%	1 Missing ⚠️
...gle/cloud/storage/internal/storage_stub_factory.cc	91.66%	1 Missing ⚠️
...loud/storage/internal/storage_stub_factory_test.cc	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16170      +/-   ##
==========================================
- Coverage   92.21%   92.20%   -0.02%     
==========================================
  Files        2265     2268       +3     
  Lines      209121   209317     +196     
==========================================
+ Hits       192835   192994     +159     
- Misses      16286    16323      +37     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.