queryserving, observability: instrument vttablet query cache plan hits/misses

[maxenglander]

Description

Add stats counters:

• QueryCacheHits: Query engine query cache hits
• QueryCacheMisses: Query engine query cache misses

A query plan cache miss results in VTTablet parsing the query. This can add a significant amount of latency to a query execution. Having better visibility into query plan caching will help isolate or eliminate cache misses as a source of latency when analyzing query performance.

Related Issue(s)

Fixes #14946

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added: content/en/docs/19.0: document QueryCache{Hits,Misses} stats website#1663

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (696fe0e) 67.41% compared to head (0ff0a63) 67.44%.
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14947      +/-   ##
==========================================
+ Coverage   67.41%   67.44%   +0.02%     
==========================================
  Files        1560     1561       +1     
  Lines      192752   193194     +442     
==========================================
+ Hits       129952   130301     +349     
- Misses      62800    62893      +93     

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[frouioui]

This should go in the changelog/20.0/20.0.0/summary.md summary file, if it does not exist yet you can create it.

[frouioui]

Also a small nit pick, reading this it seems like VTTablet is only exposing these two counter stats now, maybe add "new" or something

[frouioui]

LGTM but the release notes must be changed. 🥳

[frouioui]

Still LGTM

[frouioui]

Conflicts are resolved, should be merged soon!