Monitoring: Backup duration and backup set size

Closes privatestorageops#429

See merge request !339

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

Backup: Borg: All logs as JSON please

See merge request !338

That makes working with its output in Grafana easier.
Good documentation can be found at
https://borgbackup.readthedocs.io/en/stable/internals/frontends.html#json-output

Backup: Add --stats to borgbackup create options

See merge request !337

We want to have this in our own monitoring

Monitoring: Backup: Clearer coloring

See merge request !336

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

Refs privatestorageops#429.

Disregard ZFS ARC cache when monitoring free RAM

Closes #119

See merge request !331

Monitoring: Add a "Node Exporter Full" dashboard

See merge request !332

, a dashboard that "displays a lot of data about one single host".

This is

One query for hosts with ZFS and one for those without.

Since ZoL frees ARC under memory pressure, let's not
count it as "used" but instead as "free" memory.

Set minimal nixos profile for our grid machines

See merge request !330

First version of borgbackup monitoring dashboard and alerts

See merge request !323

bump nixpkgs version

See merge request !327

Bump megacli version to fix exception handling

See merge request !321

Configure production grid monitoring to also use new 2nd level domain

See merge request !324

refs privatestorageops#416, PrivateStorageio#115

Automatically create production update MRs

See merge request !318

bump nixpkgs version

See merge request !320