Monitoring: Backup duration and backup set size

Fixes privatestorageops#429

One dash for backup set size

• Unfortunately still a bit buggy, should be resolved in later Grafana (?)
• Let's see how it looks with more than one host (sorry, on production only) ...

One dash for actual backup durations on the vertical (y) axis

• Gets daily user ciphertext backup duration in seconds from Loki, transforms to number format, and graphs
• Paint green from 0s, then yellow from 3600s (reason: that's a long time), then red from 10800s (reason: that's the first chance it could collide with the borgbackup-check-repo job)
• Hopefully looks nice and gives quick info in absolute numbers
• "duration" input sadly not available for borgbackup "check" job. That's why we also have:

Two dashes that graph the systemd unit's job states on the horizontal (x) axis:

• Shows backup jobs being active over time
• Nice to see if jobs overlap (not great if all hosts run backups at the same time)
• Using Prometheus -> comes with alerting if backup job state is "active" for too long

Screenshot