Multi-metrics throttler post v21 cleanup: remove unthrottled entry from topo

[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 is a small extract of #16915, addressing a single issue: when an app is unthrottled (or expired), we now remove its configuration from topo's Keyspace record, as opposed to leaving it there with zero values.

This helps maintaining the topo record small and tidy. This could not have been done in v21 for compatibility reasons: the throttler maintains an in-memory representation of the config. Up to v20, it would not remove entries from its in-memory represenation even if those entries did not exist in the config. It would only overwrite based on existing config entries. v21 knows better and removes such entries, which is why it's only safe to submit this PR in v22.

Related Issue(s)

#15624

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.

Project coverage is 67.41%. Comparing base (ab7b516) to head (de4c1f8).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/schemamanager/tablet_executor.go	0.00%	6 Missing ⚠️
go/vt/vtctl/grpcvtctldserver/server.go	0.00%	6 Missing ⚠️
go/vt/vtgate/executorcontext/vcursor_impl.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17283   +/-   ##
=======================================
  Coverage   67.41%   67.41%           
=======================================
  Files        1576     1576           
  Lines      253417   253408    -9     
=======================================
- Hits       170846   170840    -6     
+ Misses      82571    82568    -3     

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[mattlord]

❤️ This will make it much easier to understand the config.