Tablet throttler: adding more stats

[shlomi-noach]

Description

Closes #15223 by adding these counter metrics:

• ThrottlerHeartbeatRequests: incremented each time the throttler requests heartbeat lease (especially useful when --heartbeat_on_demand_duration is set).
• ThrottlerRecentlyChecked: for when a tablet's throttler is recently checked. This applies to any tablet type. For example, a vreplication rowstream could run on a particular replica. That replica would report itself as being "recently checked".
• ThrottlerProbeRecentlyChecked: for when a PRIMARY tablet throttler probes the shard tablets, and one of the probed tablets reports it has been "recently checked".
• ThrottlerCheckRequest: when a tablet receives a gRPC CheckThrottler call.

These metrics will help to pinpoint throttler behavior, and better understand the flow of requests.

Related Issue(s)

• Feature Request: add more visibility/stats in Tablet Throttler #15223
• Docs PR: Docs: throttler metrics listing and breakdown website#1690

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

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[mattlord]

We should document the throttler metrics either here: https://vitess.io/docs/19.0/reference/features/tablet-throttler/#information or in a new #monitoring section.

For example: https://vitess.io/docs/19.0/reference/vreplication/metrics/ and vitessio/website#1687

[shlomi-noach]

Docs PR: vitessio/website#1690