VTGate Warnings: Add WarnUnshardedOnly to warnings counter

[tycol7]

Description

When running VTGate with the --warn_sharded_only flag, increment the warnings count when executing a query that is supported in an unsharded keyspace but not supported in a sharded one.

Useful for reporting purposes.

Related Issue(s)

• Produce query warnings for using features not supported when sharded #7538

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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2162883) 47.50% compared to head (20adf7e) 47.50%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15033   +/-   ##
=======================================
  Coverage   47.50%   47.50%           
=======================================
  Files        1149     1149           
  Lines      239324   239325    +1     
=======================================
+ Hits       113682   113694   +12     
+ Misses     117053   117044    -9     
+ Partials     8589     8587    -2     

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[deepthi]

Looks fine to me. I'll let @harshit-gangal say whether this needs a separate issue.
This PR will most likely conflict with #15010 from @maxenglander. Whichever of these is merged first, the other one will need conflict resolution before merge.

[harshit-gangal]

I merged the other one. This need conflict resolution.