Multi-metrics throttler: post v21 deprecations and changes

[shlomi-noach]

Description

Multi metrics throttler was introduced in v21 and was backwards compatible with the v20 single-metric throttler. Compatibility included gRPC, command line flags, etc.

Starting v22, we remove single-metric throttler compatibility. This PR implements various topics seen in:

• RFC: tablet throttler multi-metrics #15624 (comment)
• RFC: tablet throttler multi-metrics #15624 (comment)

Changes in this PR:

• Remove MultiMetricsEnabled and its usage. It is assumed to be true in v22.
• Remove expired/unthrottled rules from topo rather than set them to 0 expiration duration
• Remove --check-as-check-self and --cehck-as-check-shard command line flags and all related implementation.
• Remove all HTTP StatusCode usage.

Related Issue(s)

• RFC: tablet throttler multi-metrics #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]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 35.89744% with 25 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (cb66920) to head (4aab771).
Report is 57 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletserver/tabletserver.go	0.00%	15 Missing ⚠️
go/vt/vtctl/vtctl.go	0.00%	6 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_throttler.go	0.00%	3 Missing ⚠️
go/vt/vttablet/tabletserver/throttle/check.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16915      +/-   ##
==========================================
- Coverage   67.49%   67.47%   -0.02%     
==========================================
  Files        1577     1577              
  Lines      253416   253356      -60     
==========================================
- Hits       171045   170960      -85     
- Misses      82371    82396      +25     

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[mattlord]

We don't want to have a require.NoError(t, checkErr) here before the defer, getting rid of the conditional check? Same below.

[shlomi-noach]

Not in this case, because this is a wait-for function, and I'm OK ignoring the error as long as the function eventually succeeds within the given timeout. So I don't care that there could be multiple errors, and I don't want to report the test as failed.

[mattlord]

Do we care about reserving the name too? I'm not sure, just asking. We can always do that later since your comment clearly indicates what the name was. It's also fine to eventually reuse field names, as long the json/text encodings are still usable and sensible.

[shlomi-noach]

Added reserved names.

[shlomi-noach]

Converted to draft while reviewing newly merged throttler PRs.

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