Collects the smart data using smartctl and outputs them in the
textcollector dir. This expects smartd to be configured to regularly
self tests on a regular interval to detect if a disk is broken.

prometheus-borg-textcollector is no longer started by timer, but instead
defines a WantedBy= relationship with the borg-backup{,-offsite} service.

Collect prometheus btrfs errors from the btrfs command from btrfs-progs
which since 5.10 supports json output for device stats. The collected
errors will in the future trigger an alert when the errors reach a
certain treshold.

To monitor our archive mirrors and the archive size itself a new
textcollector has been added. This will allow us to monitor the archive
growth and the sync rate to mirrors.