Record the rebuilderd queue length in prometheus so we can generate an
alert for when the queue length keeps rising. As this could be an
indication that the rebuilders have builds which are stuck.

Run the blackbox exporter on monitoring.archlinux.org to monitor other
machines http status for public services we provide. Also has an alert
for when a certificate is about to expire in 3 days.

Add a new role called prometheus_exporters which should be run on every
machine we have and starts different collectors depending on what group
the machine is in. Currently supported our the gitlab runner exporter,
rebuilder textcollector, mysqld-exporter, borg textcollector and an
node/arch exporter. The arch exporter monitors the security status and
pacman out of date packages gauge.