Add Deferred Optimization Execution Metric

[harshit-gangal]

Description

This PR introduces a new metric to track the execution of Deferred Optimization in Vitess. Deferred Optimization enhances query planning by generating a more efficient plan during the first execution of a prepared statement.
The new metric helps monitor how often queries leverage this feature, providing insights into its impact.

Related Issue(s)

• Implement Deferred Optimization for Prepared Statements #17992
• Feature Request: Prepared Statement Planning Improvement #17840

Checklist

• 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

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

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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

Attention: Patch coverage is 57.44681% with 20 lines in your changes missing coverage. Please review.

Project coverage is 67.54%. Comparing base (b0352bd) to head (9fa5bd8).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/executorcontext/vcursor_impl.go	32.14%	19 Missing ⚠️
go/vt/vtgate/engine/plan_switcher.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18067      +/-   ##
==========================================
- Coverage   67.61%   67.54%   -0.08%     
==========================================
  Files        1600     1601       +1     
  Lines      260810   261330     +520     
==========================================
+ Hits       176351   176516     +165     
- Misses      84459    84814     +355     

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