Feature Request: MySQL load average & used disk space metrics in multi-metric throttler

[shlomi-noach]

Feature Description

Context: #15624

We wish to add these two metrics:

• Load average on the MySQL host. We already have the loadavg metric, but that is read on the vttablet host. The two are not necessarily the same.
• Disk space used for the MySQL datadir. We may want to throttle e.g. massive schema migrations when running out of disk space for either table copy or for binlog events.

To this effect we need new message + request + response types in mysqlctl.proto.