Add statement SYSTEM RELOAD ASYNCHRONOUS METRICS

[rschu1ze]

This PR was born out of an annoyance with integration tests where it was necessary to prepend SELECT * FROM system.asynchronous_metrics with sleep(sth > asynchronous_metrics_update_interval) to make sure the system tables are up-to-date.

Commits:

• Some cosmetics 1262945
• Refactor the locking (it was a bit weird before) into two mutexes 748371d
• Add SYSTEM RELOAD ASYNCHRONOUS METRICS 90a0ea3

Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Added statement SYSTEM RELOAD ASYNCHRONOUS METRICS which updates the asynchronous metrics. Mostly useful for testing and development.

[robot-ch-test-poll2]

This is an automated comment for commit 8fc918b with description of existing statuses. It's updated for the latest CI running

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[alexey-milovidov]

Ok.

Caveats:

• there are AsynchronousMetrics and heavier, ServerAsynchronousMetrics (both need to be updated).
• take care of AsynchronousMetricsUpdateInterval and Jitter metrics.

[antonio2368]

there are AsynchronousMetrics and heavier, ServerAsynchronousMetrics (both need to be updated).

Not sure I understand this part, but AsynchronousMetrics is just base class for ServerAsynchronousMetrics which will call all updates in update, both from base class and from derived.
Server defines only one object for async metrics.

[rschu1ze]

I refreshed this PR after a long time. Alexey's points are addressed.

@antonio2368 maybe you like to take a look? (I guess this PR appears so far in the open PR list on GitHub that nobody will assign 😄)

[antonio2368]

you can make a much better test
set asynchronous_metrics_update_period_s to a really large value
and then pick a metric, e.g. Uptime or NumberOfTables to check in system.asynchronous_metrics it's updated after this call

[rschu1ze]

Done, thanks.

[antonio2368]

now I see what Alexey was saying
In server metrics there is updateHeavyMetricsIfNeeded which has a different update interval.
They won't be updated with this call unless the interval has passed since last update.
I think you need to add a new argument bool force to update and updateImpl which will ignore such checks.

[rschu1ze]

Right, thanks. Updated.

[rschu1ze]

There were Tsan issues and weird results (probably due to races) in previous commits.
These should be gone with the reworked locking.