Merge branch 'add-project_remote_mirror_last_update_time_seconds' into 'master'

Add project_remote_mirror_last_successful_update_time_seconds metric

See merge request gitlab-org/gitlab-monitor!91
2 jobs for v3.7.0 in 51 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #4320392
rspec

00:00:51

passed #4320393
rubocop

00:00:50