Tablet throttler: non-PRIMARY tablets report back to PRIMARY throttler when they've been 'check'ed

[shlomi-noach]

Description

#13175 describes a starvation scenario, where a streamer happens to read from a REPLICA and nothing on the PRIMARY checks the throttler. This may happen when:

• using --heartbeat_on_demand_duration
• as mentioned, streamer reads from non-PRIMARY

and when:

• The writes happen on another cluster (e.g. MoveTables, resharding)
• or, the writes happen on the same cluster, but the writer in charge does not consult the throttler.

Up till now, throttler checks on the replica would not trigger on-demand heartbeats on the source cluster. This can lead to a "starvation" scenario, where the app is continuously throttled: nothing writes heartbeats on the source cluster. the checks on the replica do not trigger heartbeats, the throttler thinks there's lag, stalls reads, no writes taking place, the entire flow is at standstill.

As of this PR, a replica's throttler service takes note when someone (other than the throttler mechanism itself) performs a check. It notes down "I have been recently checked". This note expires between 1sec and 2sec.

As the PRIMARY throttler probes the replicas, it also collects this new information. If it sees a replica which has "just been recently throttled", then it asks for a heartbeat lease renewal.

With this PR, any check on the throttler of any tablet in the cluster, will trigger a heartbeat lease renewal.

Related Issue(s)

• Fixes VReplication Workflows and VStream API gets stuck in the copy phase if tablet type is set only as a replica #13175
• Suggestion: Vttablet: on demand lag throttler heartbeats to reduce binary log volume #10196

Checklist

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• Documentation was added or is not required

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[shlomi-noach]

Implicitly merged via #13195