Generalize non-streaming result stats aggregation

[rafer]

Description

Introduces Result#MergeStats and ensures that the non-streaming versions (#TryExecute) of all Primitives return aggregated stats for any underlying wrapped primitives. MergeStats only handles RowsAffected at the moment, but extracting this function and making sure it is called on wrapped primitives makes it easier for downstream forks to add additional result-level stats (by updating MergeStats), and ensures that they correctly are propagated (and not dropped).

This is an incremental improvement, and does not attempt to implement stats aggregation in #TryStreamExecute.

Related Issue(s)

#16481

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

N/A

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 93.22034% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.68%. Comparing base (59408f9) to head (fcda667).
Report is 86 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/engine/insert_select.go	81.81%	2 Missing ⚠️
go/vt/vtgate/engine/sequential.go	0.00%	1 Missing ⚠️
go/vt/vtgate/engine/upsert.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16480      +/-   ##
==========================================
+ Coverage   68.64%   68.68%   +0.04%     
==========================================
  Files        1551     1551              
  Lines      199471   199719     +248     
==========================================
+ Hits       136925   137178     +253     
+ Misses      62546    62541       -5     

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[systay]

slapping the Do Not Merge label while we are discussing alternative solutions to this

[rafer]

Closing in favor of #16638