Can lose seconds/minutes of data on controller/labelgroup termination

[trent-s]

The SusQL controller stores data in the etcd database and in the SusQL Prometheus database. Both are updated regularly, yet when the controller restarts or a labelgroup is deleted, unlogged data is lost. Seconds of data lost may be acceptable for very long running jobs, but we should look carefully for ways to minimize the period of potential data loss.