Throttler multi-metrics: an incremental PR

[shlomi-noach]

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 https://github.com/vitessio/vitess/pull/15988into 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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[shlomi-noach]

We ended up deciding to review #15988 as a while. Closing this PR to avoid confusion.