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

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

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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 25.00000% with 42 lines in your changes missing coverage. Please review.

Project coverage is 69.39%. Comparing base (f40e076) to head (ef5ccc5).
Report is 1 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/schemamanager/tablet_executor.go	0.00%	5 Missing ⚠️
go/vt/vtctl/grpcvtctldserver/server.go	0.00%	5 Missing ⚠️
go/vt/vtgate/vcursor_impl.go	0.00%	5 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_throttler.go	0.00%	3 Missing ⚠️
.../tabletserver/throttle/base/self_metric_loadavg.go	0.00%	2 Missing ⚠️
go/vt/vttablet/tabletserver/throttle/check.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16915      +/-   ##
==========================================
- Coverage   69.43%   69.39%   -0.04%     
==========================================
  Files        1570     1570              
  Lines      203812   203811       -1     
==========================================
- Hits       141517   141435      -82     
- Misses      62295    62376      +81     

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