dbconsole: drain should show up in event log

**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