go/vt/vtgate: add mirror query stats to vtgate /debug/querylogz

[maxenglander]

Description

Expose mirror query stats in VTGate /debug/querylogz output.

Related Issue(s)

Closes #17697.

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 or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

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Documentation

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• There should be a comment at the top of each new or modified test to explain what the test does.

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Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.80%. Comparing base (71b2cae) to head (1706774).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17698      +/-   ##
==========================================
+ Coverage   67.77%   67.80%   +0.02%     
==========================================
  Files        1587     1587              
  Lines      255798   255803       +5     
==========================================
+ Hits       173379   173445      +66     
+ Misses      82419    82358      -61     

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