feat(metrics): keep read_bytes_count unchanged and add experimental_read_bytes_count and transition tracking

[kislaykishore]

Description

Reverted the gcs/read_bytes_count metric to stay unchanged (zero attributes, original description). Added a new attributed metric gcs/experimental_read_bytes_count tracking read bytes categorized by read_type attributes (Parallel, Random, Sequential, Unknown).

Additionally, introduced sequential-to-random read access transition reasons tracking:

• Added a new custom counter metric gcs/experimental_sequential_to_random_transitions_count mapped to transition reasons: backward_seek, forward_seek, and initial_offset_non_zero.
• Instrumented the read type classifier (ReadTypeClassifier) to dynamically classify and atomically switch state (using thread-safe CompareAndSwap) on sequential to random transitions.
• Integrated a new mock-based unit test validation suite inside read_type_classifier_test.go and updated high-level integration test assertions in both gcs_metrics_test.go and metrics_test.go.

Link to the issue in case of a bug fix.

N/A (At user's direct request)

Testing details

1. Manual - Ran unified compile, code generation, import formatting, vet, and style lint pipeline (make build) successfully with 0 warnings/issues.
2. Unit tests - Executed unit test sweeps under metrics package (go test -v ./metrics/...) and gcsx package (go test -v ./internal/gcsx/... -run TestReadTypeClassifier) completely successfully.
3. Integration tests - Executed file system integration test suite (go test -v ./internal/fs/... -run "TestGCSMetrics|TestReadFile"), fully verifying metrics behaviors under file cache hit/miss cycles, sequential readers, buffered prefetch pipelines, and parallel MRD downloads.

Any backward incompatible change? If so, please explain.

N/A (Reverted gcs/read_bytes_count back to its original signature, making it fully backwards-compatible for all active telemetry consumers).

[codecov]

Codecov Report

❌ Patch coverage is 84.33735% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.75%. Comparing base (11ac2fc) to head (2b400bd).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
metrics/metrics_test_utils.go	0.00%	7 Missing ⚠️
internal/gcsx/random_reader.go	85.00%	5 Missing and 1 partial ⚠️
internal/gcsx/inactive_timeout_reader.go	66.66%	2 Missing and 3 partials ⚠️
internal/gcsx/client_readers/range_reader.go	90.69%	4 Missing ⚠️
internal/gcsx/read_type_classifier.go	90.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4720      +/-   ##
==========================================
+ Coverage   83.70%   83.75%   +0.05%     
==========================================
  Files         168      168              
  Lines       20740    21057     +317     
==========================================
+ Hits        17360    17636     +276     
- Misses       2734     2772      +38     
- Partials      646      649       +3     

Flag	Coverage Δ
unittests	83.75% <84.33%> (+0.05%)	⬆️

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