feat(storage): add OpenTelemetry counters for read hedging

[ajayky-os]

Add storage.read_hedging.hedges_dispatched and storage.read_hedging.hedge_won counters to track speculative attempts and secondary hedge wins.

[gemini-code-assist]

Code Review

This pull request integrates OpenTelemetry metrics tracking for read hedging operations in HedgedObjectReadSource, introducing a new HedgedReadMetrics helper class. The review feedback highlights several style guide violations and optimization opportunities, including avoiding auto when it obscures return types, declaring loop variables in the narrowest scope, removing unnecessary braces for short if statements, and passing nullptr instead of creating a new metrics instance in the delegating constructor to avoid performance overhead.

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.28%. Comparing base (4200a62) to head (54f0996).

Files with missing lines	Patch %	Lines
google/cloud/storage/internal/connection_impl.cc	0.00%	4 Missing ⚠️
...loud/storage/internal/hedged_object_read_source.cc	96.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16410      +/-   ##
==========================================
+ Coverage   92.25%   92.28%   +0.03%     
==========================================
  Files        2246     2246              
  Lines      212528   212668     +140     
==========================================
+ Hits       196060   196259     +199     
+ Misses      16468    16409      -59     

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[ajayky-os]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces OpenTelemetry metrics for read hedging operations by adding a new HedgedReadMetrics class and integrating it into HedgedObjectReadSource and StorageConnectionImpl. The review feedback suggests optimizing the implementation by passing std::shared_ptr<HedgedReadMetrics> directly and using null checks instead of allocating dummy metrics on the heap when telemetry is disabled or not provided.