Tablet throttler: get remote tablets metrics from Realtime Stats , with auto-detection

[shlomi-noach]

Description

An enhancement of #13018 ; per #13018 (comment), this is a modification where the newly introduced --feature-throttler-read-realtime-stats command line flag is not required, and removed in this PR.

In this PR we track availability of throttler metrics in RealtimeStats. If a throttle metric was seen in RealtimeStats in the past minute, we do not run probes on the relevant tablet. If no metric has been seen for a tablet in the past minute, then the throttler runs the usual probes (currently HTTP based) for that tablet.

I'm not sure this approach is better than #13018, and the reason has to do with probe frequency. The PRIMARY tablet runs the standard probes run in subsecond intervals. However, it has no control over the probing frequency in other tablets. Thus, if --health_check_interval is high on replica tablets, say 10s, that means the PRIMARY has low resolution for throttler metrics (in particular, replication lag).

It does make sense when the throttler's threshold accommodates --health_check_interval. For example, health_check_interval of 5s makes sense if throttler is configured for the default replication lag metric, and the threshold is configured to, say, 30. But if the threshold is at 5s, then I'd expect a 1s-2s value for health_check_interval.

Related Issue(s)

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This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

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[shlomi-noach]

We're not going to pursue this path. Instead, we will convert throttler's HTTP calls with RPC calls.