go/vt/mysqlctl{/backupstats}: throttle backupstats for perf

[maxenglander]

Description

#11979 added detailed backup/restore metrics, but appeared to add ~4% overhead in a local test. I'm no longer able to reproduce the findings of that test.

In spite of that, this PR attempts to reduce any overhead by throttling stats calls.

Takes inspiration from logutil.ThrottledLogger, but does not drop any stats. Instead batches and and combines unflushed stats and then flushes them either when Flush() is called by the user, or a subsequent stats call occurs after a user-specified time interval.

Performance

A repeat of the performance test in #11979.

I set up the commerce example cluster, and created a table with ~30 GiB of data by repeatedly inserting REPEAT(UUID(),10), and then ran...

$ vtctlclient Backup -- --allow_primary <tablet>

...multiple times, collecting the value of vttablet_backup_duration_seconds.

Did this with the main commit before backupstats was introduced, the main commit when backupstats was introduced, and the HEAD of this PR.

#	main (7fc1b48)	main (d5364e6)	this PR (65ac8fe2)
1	89	90	89
2	89	90	89
3	90	89	88
4	91	89	90
5	89	89	91
Δ	N/A	-0.22%	-0.22%

In the last PR I found that d5364e6 added a bit of overhead 7fc1b48.

adds a roughly 3.8% performance overhead.

In this PR, it's not evident that d5364e6 adds any overhead. It's also not evident that 65ac8fe2 is any improvement to d5364e6.

I'm not sure it makes sense to merge this code, since it's not clear that there's anything to fix.

One possible explanation for getting misleading results in the last PR. I used BackupShard in the last PR whereas I'm using Backup against a specific tablet in this PR. Correct me if I'm wrong but I think that BackupShard picks a random replica tablet. Maybe small differences between replicas could result in different performance results from runs against randomly selected replicas.

Related Issue(s)

#11977
#11979

Checklist

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[deepthi]

If there is no difference in performance, I'd like us to hold off on merging this PR.
Maybe when you implement the same metrics for s3, you can re-do the benchmarks, and let's see if this makes a difference?

[maxenglander]

Returning this PR to draft until we have compelling evidence that it's needed.

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