feat(storage): add telemetry for pre-warmed ranges in ObjectDescriptorImpl

[kalragauri]

This PR adds OpenTelemetry instrumentation for multi-range pre-warming requests in the GCS async client.

[gemini-code-assist]

Code Review

This pull request introduces tracking and telemetry for the cache status ("HIT", "MISS", "EVICTED") of pre-warmed ranges in ObjectDescriptorImpl, recording it as an OpenTelemetry span attribute. The review feedback suggests limiting the size of the new evicted_ranges_ set to prevent unbounded memory growth, and optimizing performance by using absl::string_view instead of std::string to avoid unnecessary copies of the cache status.

[codecov]

Codecov Report

❌ Patch coverage is 80.45977% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (c6a56cb) to head (d3803bb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/storage/internal/async/object_descriptor_impl.cc	34.61%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16323      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.01%     
==========================================
  Files        2227     2227              
  Lines      209208   209283      +75     
==========================================
+ Hits       192976   193030      +54     
- Misses      16232    16253      +21     

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