Throttler: SelfMetric interface, simplify adding new throttler metrics

[shlomi-noach]

Description

An internal cleanup/refactor. Intorducing no change in behavior.

This PR refactors how the throttler reads and treats the multi-metrics (lag, custom, threads_running, loadavg) by introducing the SelfMetric interface with implementing structs

This makes it easy to add new metrics: create a new self_metric_<name>.go, implement SelfMetric, register the metric, and you're good. The throttler will invoke the Read function for all registered metrics.

The PR cleans up even more code from the throttler, eliminating MySQLMetricConfigurationSettings., removing throttler's metricsQuery, and more.

Related Issue(s)

• RFC: tablet throttler multi-metrics #15624 (comment)
• Followup to Tablet throttler: multi-metric support #15988

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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[shlomi-noach]

File was just renamed

[shlomi-noach]

This file was mostly just renamed, but also some lines of code moved to other files. There isn't anything important to review here.

[shlomi-noach]

File just renamed

[shlomi-noach]

New file, with some code extracted from elsewhere, but otherwise introducing nothing new. Nothing to review here.

[shlomi-noach]

New file, with code moved from other files, but introducing nothing new. Nothing to review here.

[codecov]

Codecov Report

Attention: Patch coverage is 50.24155% with 103 lines in your changes missing coverage. Please review.

Project coverage is 68.63%. Comparing base (e341f23) to head (edea801).
Report is 1 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletserver/throttle/throttler.go	24.24%	25 Missing ⚠️
...vttablet/tabletserver/throttle/base/self_metric.go	34.28%	23 Missing ⚠️
.../tabletserver/throttle/base/self_metric_loadavg.go	25.00%	18 Missing ⚠️
...tablet/tabletserver/throttle/base/metric_result.go	54.16%	11 Missing ⚠️
...blet/tabletserver/throttle/base/self_metric_lag.go	50.00%	8 Missing ⚠️
...vttablet/tabletserver/throttle/base/metric_name.go	86.11%	5 Missing ⚠️
.../tabletserver/throttle/base/self_metric_default.go	54.54%	5 Missing ⚠️
...etserver/throttle/base/self_metric_custom_query.go	60.00%	4 Missing ⚠️
...erver/throttle/base/self_metric_threads_running.go	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16469      +/-   ##
==========================================
+ Coverage   68.62%   68.63%   +0.01%     
==========================================
  Files        1551     1558       +7     
  Lines      199515   199550      +35     
==========================================
+ Hits       136915   136963      +48     
+ Misses      62600    62587      -13     

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[shlomi-noach]

Ready for review

[shlomi-noach]

The parent for all metric types. Implement this to add a new metric.

[mattlord]

Looks good! I only had a few minor suggestions/nits that you can resolve as you feel best.