Docs: throttler metrics listing and breakdown

[shlomi-noach]

This adds a Metrics section to the throttler docs, for both v20 and v19. It lists all the metrics the throttler exports (including these new metrics for v20). It also discusses the use case for monitoring some of these metrics.

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[shlomi-noach]

Relevant previews:

• https://deploy-preview-1690--vitess.netlify.app/docs/20.0/reference/features/tablet-throttler/#metrics
• https://deploy-preview-1690--vitess.netlify.app/docs/19.0/reference/features/tablet-throttler/#metrics

[shlomi-noach]

Resubmitted in https://github.com/vitessio/website/pull/1694