feat(metrics): Adding the metadata_cache read count metric

[thrivikram-karur-g]

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Description

Adding a metric to capture the metadata cache reads.
Hits & misses, more detail on the cache lookup and if cached entry is positive or negative.

Link to the issue in case of a bug fix.

b/512002923

Testing details

1. Manual - NA
2. Unit tests - NA
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

N/A

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request adds observability to the metadata cache by implementing a new read count metric. By tracking cache hits and misses along with the status of entries, this change provides deeper insights into cache efficiency and lookup behavior, which is critical for debugging and performance tuning in the GCSFuse file system.

Highlights

• Metric Implementation: Introduced the metadata_cache/read_count metric to track cache lookup performance, including hits, misses, and entry status (positive/negative).
• StatCache Integration: Updated StatCacheBucketView to accept a MetricHandle, enabling instrumentation of cache lookups within the metadata cache layer.
• Testing and Validation: Added comprehensive unit and integration tests to verify metric accuracy across various cache scenarios, including TTL expiration and negative cache hits.

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[gemini-code-assist]

Code Review

This pull request integrates metadata cache read count metrics tracking into gcsfuse. It updates NewStatCacheBucketView to accept a metrics.MetricHandle and records cache hits, misses, entry statuses (positive/negative), and lookup details (found, not found, TTL expired). Additionally, it updates existing tests to use no-op metrics, adds comprehensive unit and integration tests for the new metrics, registers the "metadata_cache/" prefix, and fixes formatting in test utility assertion failures. There are no review comments, so no feedback is provided.