Throttler multi-metrics: an incremental PR

[shlomi-noach]

Replaced by #16039

Notes

This PR is the incremental version of #15988, and will be the base of multiple coming up PR merges. DO NOT MERGE

Description

This PR adds multi-metrics support to the tablet throttler. It is pretty large as will be explained shortly, and is submitted as Draft. I will break it down into hopefully smaller and more manageable PRs, something I could not do in the process of writing this code. To explain a bit, adding multi-metrics support adds a new dimension of complexity to the already multi-dimensioned throttler. I chose do remove legacy or unused dimensions from the existing codebase so as to simplify the result. While this PR is not a redesign (the main elements remain the same), it shaves and refactors a lot of code.

See full changes and documentation in #15988 . We are breaking down #15988 into smaller changes which will make it easier to review. This changes will merge onto this branch, with the intention that at the end of the process this PR/branch will be (mostly) identical to #15988 , at which time it will be merged as a whole.

There will only be a handful of PRs that we will merge directly to main. These will be some cleanup-related changes, bugfixes, CI fixes, etc. But the multi-metric redesign will merge into this branch and then as a whole into main. There is a lot of backwards-compatibility and testing involved, and I feel the complete PR is correct to merge as a whole.

Related Issue(s)

#15624

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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.25%. Comparing base (d95ce48) to head (e9b68ee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16012      +/-   ##
==========================================
- Coverage   68.27%   68.25%   -0.02%     
==========================================
  Files        1562     1562              
  Lines      197319   197319              
==========================================
- Hits       134714   134689      -25     
- Misses      62605    62630      +25     

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[shlomi-noach]

This PR is now closed, replaced by #16039

For logistics reasons, we will use #16039 as the icnremental PR into which we merge further multi-metric throttler changes