feat(metrics): Adding streaming write fallback count metric

[thrivikram-karur-g]

Description

Adding streaming write fallback metrics that tracks the reason for the fallback and also which file mode & flag it fell back to staging writes. Useful for understanding why streaming writes fallback hasn't been picked .

Link to the issue in case of a bug fix.

b/499889281

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

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This pull request introduces observability into the streaming write fallback mechanism within the file system. By adding a new counter metric, it enables better monitoring and analysis of why streaming writes may not be utilized, helping to identify performance bottlenecks or configuration issues related to write operations.

Highlights

• New Metric Added: Introduced the 'fs/streaming_write_fallback_count' metric to track occurrences where streaming writes fall back to legacy staged writes.
• Fallback Reason Tracking: Implemented detailed tracking for various fallback reasons, including out-of-order writes, existing files, concurrent limit breaches, and configuration settings.
• Open Mode Context: Added support for recording the file's open mode (e.g., read-write, write-only) alongside the fallback reason to provide better context for debugging.

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[gemini-code-assist]

Code Review

This pull request introduces a new metric, fs/streaming_write_fallback_count, to track when the system falls back from streaming writes to legacy staged writes. The changes include updating the FileInode struct and its methods to record these fallbacks with specific reasons such as out-of-order writes, concurrent limits, or existing files. Additionally, the metrics package was updated with new types, constants, and OTel test coverage. One review comment points out a discrepancy in the documentation where the metric is described as a 'cumulative distribution' despite being defined as a counter in the YAML configuration.

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.70%. Comparing base (8a9f498) to head (7c2690d).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/fs/inode/file.go	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4630      +/-   ##
==========================================
+ Coverage   83.64%   83.70%   +0.06%     
==========================================
  Files         164      164              
  Lines       20272    20439     +167     
==========================================
+ Hits        16956    17109     +153     
- Misses       2676     2690      +14     
  Partials      640      640              

Flag	Coverage Δ
unittests	83.70% <96.77%> (+0.06%)	⬆️

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