chore(fracmanager): extract CacheMaintainerMetrics to separate file

[eguguchkin]

Fixes #150

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• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.52632% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.18%. Comparing base (6ffb9f1) to head (d87cb5c).

Files with missing lines	Patch %	Lines
cmd/index_analyzer/main.go	0.00%	8 Missing ⚠️
fracmanager/cache_maintainer_metrics.go	93.33%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   71.22%   71.18%   -0.04%     
==========================================
  Files         198      199       +1     
  Lines       18210    18223      +13     
==========================================
+ Hits        12970    12972       +2     
- Misses       4522     4533      +11     
  Partials      718      718              

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[dkharms]

So let's clarify this. We decided to follow this approach? Place metrics as near as possible to the place where they are used but in the separate file with name metrics.go?