`mysqld` system metrics, with `TabletManager` rpc

[shlomi-noach]

Description

Addresses https://github.com/vitessio/vitess/issues/16822. This is part of the multi-metrics throttler project.

Part 1, to be included in v21:

• Adding mysqlctl.SystemMetrics 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.MysqlSystemMetrics, 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.SystemMetrics RPC.
• Likewise, TabletManagerClient supports MysqlSystemMetrics() 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)

• https://github.com/vitessio/vitess/issues/16822
• https://github.com/vitessio/vitess/issues/15624

Checklist

• [x] "Backport to:" labels have been added if this change should be back-ported to release branches
• [x] 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?
• [x] Documentation was added or is not required

Deployment Notes

[vitess-bot[bot]]

Review Checklist

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Backward compatibility

• [ ] Protobuf changes should be wire-compatible.
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• [ ] RPC changes should be compatible with vitess-operator
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• [ ] vtctl command output order should be stable and awk-able.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 42.10526% with 55 lines in your changes missing coverage. Please review.

Project coverage is 69.41%. Comparing base (4058966) to head (338c52c).

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/fakemysqldaemon.go	0.00%	11 Missing :warning:
go/vt/mysqlctl/mysqld.go	71.79%	11 Missing :warning:
go/vt/vttablet/grpctmclient/client.go	0.00%	9 Missing :warning:
go/vt/vttablet/grpctmserver/server.go	0.00%	7 Missing :warning:
go/vt/vttablet/tabletmanager/rpc_query.go	0.00%	7 Missing :warning:
go/vt/mysqlctl/grpcmysqlctlserver/server.go	0.00%	2 Missing :warning:
go/vt/vtcombo/tablet_map.go	0.00%	2 Missing :warning:
go/vt/vttablet/faketmclient/fake_client.go	0.00%	2 Missing :warning:
go/vt/vttablet/tabletserver/throttle/throttler.go	84.61%	2 Missing :warning:
go/vt/vttablet/tmrpctest/test_tm_rpc.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16850 +/- ## ========================================== - Coverage 69.44% 69.41% -0.03% ========================================== Files 1571 1571 Lines 202996 203081 +85 ========================================== Hits 140977 140977 - Misses 62019 62104 +85 ```

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