txthrottler: move ThrottlerInterface to go/vt/throttler, use slices pkg, add stats

[timvaillancourt]

Description

This PR cleans up the txthrottler code by:

1. Moving the ThrottlerInterface interface used to mock *throttler.Throttler => go/vt/throttler
   • This becomes Throttler interface
   • *throttler.Throttler becomes *throttler.ThrottlerImpl
   • Some tweaks to .Log() method + Results struct to support Throttler interface
2. Use slices.Equal (vs reflect.DeepEqual) to compare cell slices
3. Register healthcheck stats in txthrottler

Related Issue(s)

I'm not sure if code cleanup needs an issue - let me know!

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

• 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

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

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 90.90909% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.59%. Comparing base (f118ba2) to head (33535a5).
Report is 204 commits behind head on main.

Files	Patch %	Lines
go/vt/throttler/throttler.go	82.35%	3 Missing ⚠️
.../vttablet/tabletserver/txthrottler/tx_throttler.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16248      +/-   ##
==========================================
+ Coverage   68.40%   68.59%   +0.19%     
==========================================
  Files        1556     1544      -12     
  Lines      195121   197978    +2857     
==========================================
+ Hits       133479   135810    +2331     
- Misses      61642    62168     +526     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.