Merge branch 'add-remote-mirrors-prober' into 'master'

Add RemoteMirrorsProber

See merge request gitlab-org/gitlab-monitor!90
2 jobs for v3.6.0 in 1 minute and 12 seconds (queued for 5 seconds)
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Status Job ID Name Coverage
  Test
passed #4317625
rspec

00:01:01

passed #4317626
rubocop

00:01:09