[release-20.0] Fix query plan cache misses metric (#16562)

[vitess-bot]

Description

This is a backport of #16562

[vitess-bot]

Review Checklist

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General

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Backward compatibility

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.66%. Comparing base (d503fa4) to head (e9d39fe).
Report is 1 commits behind head on release-20.0.

Files	Patch %	Lines
go/vt/vtgate/executor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16628      +/-   ##
================================================
+ Coverage         68.65%   68.66%   +0.01%     
================================================
  Files              1541     1541              
  Lines            197890   197890              
================================================
+ Hits             135852   135887      +35     
+ Misses            62038    62003      -35     

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