... and use a smarter Prometheus query to combine the two.

This should implement my actual intentions: Alert when
backups run for longer than 3h, and the the repo check
for more than 6h.

This is a bit buggy still in our version of Grafana, but already
nice to look at / maybe useful.

Refs privatestorageops#429

This adds alerting to the backup job duration graph:  Grafana alerting
works with systemd unit metrics, i.e. a backup job unit being "active"
for too long.  Use that fact for alerting on long-running backup jobs.

... instead of connected lines default, also with working label for host

Refs privatestorageops#429

Refs privatestorageops#429

Failed backups now have a filled red area instead of a thin
yellow line.

Refs privatestorageops#429.

Response time, probe fails and TLS expiry incl. alerts.