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

[shlomi-noach]

Replaced by vitessio#16041

Description

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

Related Issue(s)

vitessio#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

[shlomi-noach]

Replaced by vitessio#16041