dbconsole: drain should show up in event log

[nicktrav]

Is your feature request related to a problem? Please describe.

The DB Console has an event log section on the overview. When a node is drained by an operator / automation (e.g. cockroach node drain) it would be informative to display this in the event log.

Describe the solution you'd like

Surface drain events in the event log.

Describe alternatives you've considered

One alternative is to spelunk through logs looking for evidence of a drain. This is far from ideal, especially if one does not have access to the logs directly.

Jira issue: CRDB-27346

[nicktrav]

Another suggestion from the suggestion box (h/t @dt):

Add a --drain-reason flag and surface the string passed to it in the event log entry.