Tablet throttler multi-metrics incremental PR: introducing metric names and scopes

[shlomi-noach]

Description

Incremental PR in the Tablet Throttler multi-metric series of PRs as per #16012 and merging into #16012.

This PR merely introduces (and does not use yet) two noteworthy types:

• MetricName - a formalized name of a metric (lag, threads_running, etc.). There is a limited known set of names, though these are easily expandable programmatically.
• Scope - the range of tablets for which a metric is collected. This is either he local tablet (self) or the entire shard (shard). When we speak of "the entire shard" we only regard the list of tables that participate in throttler activity. By default those are primary and replica.

This PR merges into vitessio:throttler-multi-metrics-incremental, which is a Draft PR, and not into main.

https://github.com/planetscale/vitess/compare/vitessio:vitess:throttler-multi-metrics-incremental...planetscale:vitess:throttler-multi-metrics-incremental-metric-names?expand=1

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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• 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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[codecov]

Codecov Report

Attention: Patch coverage is 69.64286% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 68.26%. Comparing base (e9b68ee) to head (bdfce5a).

Files	Patch %	Lines
...blet/tabletserver/throttle/base/throttle_metric.go	69.64%	17 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           throttler-multi-metrics-incremental   #16041      +/-   ##
=======================================================================
- Coverage                                68.61%   68.26%   -0.36%     
=======================================================================
  Files                                     1562     1562              
  Lines                                   201191   197374    -3817     
=======================================================================
- Hits                                    138047   134733    -3314     
+ Misses                                   63144    62641     -503     

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[shlomi-noach]

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