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.

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[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 93 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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[harshit-gangal]

Is Execute Time and Mirror Source Execute Time same or different? Ideally a Mirror execution should not impact the query execution time.

[maxenglander]

they are measured in different places. Execute Time is measured here:

vitess/go/vt/vtgate/executor.go

Line 383 in 0bf4276

logStats.ExecuteTime = time.Since(execStart)

Mirror Source Execute Time is measure here:

vitess/go/vt/vtgate/engine/mirror.go

Line 107 in 0bf4276

sourceExecTime = time.Since(sourceStartTime)

so Mirror Source Execute Time should include but may be slightly less than Execute Time. The reason we have Mirror Source Execute Time is to have an apples-to-apples way to compare the performance of mirroring.