Create a graph and ~~/or~~ alert for WAL-E backups

After https://gitlab.com/gitlab-com/infrastructure/issues/1095 we have the metrics we wanted in Prometheus so it's time to use them now

mentioned in issue #1095 (closed)

Graph available at: https://performance.gitlab.net/dashboard/db/backups?orgId=1

Soon to be in monitor.gitlab.net

Alert can be as simple as backup counts equal 0 for a long time, call it an hour.

changed title from Create a graph and/or alert for WAL-E backups to Create a graph and{+ /or+} alert for WAL-E backups

mentioned in issue #2240 (closed)

added blocked label

This is being blocked now by https://gitlab.com/gitlab-com/infrastructure/issues/2240

removed blocked label

This is done now, alert was also imported in Prometheus

Awesome, thanks!