Add visibility for live migrations of VMs

Sometimes live migrations can cause issues, usually they don't. We should probably still have some visibility into when they happen outside of stackdriver logging.

For example, live migrations of patroni nodes:

https://console.cloud.google.com/logs/query;query=logName%3D%22projects%2Fgitlab-production%2Flogs%2Fcloudaudit.googleapis.com%252Fsystem_event%22%0A%22patroni%22;timeRange=PT4H;cursorTimestamp=2021-05-06T14:47:00.742871Z?authuser=1&project=gitlab-production

Maybe we can trigger an event log, or use a log based metric? some nodes are more interesting for us than others, like gitaly, redis and patroni.