mysqld system metrics, with TabletManager rpc

[shlomi-noach]

Description

Addresses #16822. This is part of the multi-metrics throttler project.

Part 1, to be included in v21:

• Adding mysqlctl.HostMetrics RPC, which returns a map[metric-name]metric-value, e.g. metrics["loadavg"]={...Value: 1.234}
• Mysql Daemon implements the above rpc. Can be called to supply metrics read from its local OS.
• Adding tabletmanager.MysqlHostMetrics, which is an access point to the above. TabletManager is the entity that holds a MysqlDaemon, thus is the only one able to call mysqlctl.HostMetrics RPC.
• Likewise, TabletManagerClient supports MysqlHostMetrics() function, so that anyone can query a given tablet for its MySQL OS metrics.

This is all wired in this PR, ie if you call tmclient. TabletManagerClient, you get the actual OS metrics read by MysqlDaemon.

However, for 1-version backwards compatibility, this code path is not used anywhere.

We wish to merge this in v21, and immediately follow up with a PR (designed for v22) where the tablet throttler collects these metrics. It will do so sparsely.

Related Issue(s)

• Feature Request: MySQL load average & used disk space metrics in multi-metric throttler #16822
• RFC: tablet throttler multi-metrics #15624

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]

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[shlomi-noach]

This change (and everything in this file) is a small refactor. It is not strictly required for this PR, but will be needed in part 2, so I thought I may as well do it now.

[shlomi-noach]

A fully functional TabletManagerClient implementation. Not used by anyone right now for 1-version backwards support. This is what the throttler will use in the future.

[shlomi-noach]

This is the bridge between TabletManager and MysqlDaemon. It is fully functional. But right now, no one will be calling this function, so there's no backwards-compatibility issue.

[codecov]

Codecov Report

Attention: Patch coverage is 41.23711% with 57 lines in your changes missing coverage. Please review.

Project coverage is 69.43%. Comparing base (766bc64) to head (757cada).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/mysqld.go	68.29%	13 Missing ⚠️
go/vt/mysqlctl/fakemysqldaemon.go	0.00%	11 Missing ⚠️
go/vt/vttablet/grpctmclient/client.go	0.00%	9 Missing ⚠️
go/vt/vttablet/grpctmserver/server.go	0.00%	7 Missing ⚠️
go/vt/vttablet/tabletmanager/rpc_query.go	0.00%	7 Missing ⚠️
go/vt/mysqlctl/grpcmysqlctlserver/server.go	0.00%	2 Missing ⚠️
go/vt/vtcombo/tablet_map.go	0.00%	2 Missing ⚠️
go/vt/vttablet/faketmclient/fake_client.go	0.00%	2 Missing ⚠️
go/vt/vttablet/tabletserver/throttle/throttler.go	84.61%	2 Missing ⚠️
go/vt/vttablet/tmrpctest/test_tm_rpc.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16850   +/-   ##
=======================================
  Coverage   69.43%   69.43%           
=======================================
  Files        1571     1571           
  Lines      203086   203173   +87     
=======================================
+ Hits       141013   141079   +66     
- Misses      62073    62094   +21     

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[mattlord]

LGTM! I only had some minor suggestions / questions. Let me know what you think and I'll come back to this quickly.

[mattlord]

Thanks, @shlomi-noach ! ❤️