Add alerting for repositories mirrored on github
Assigning to @nolith since he solved https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/3703. @erushton, please reassign in case there is someone more appropriate.
CC @marin to investigate if this is related to a recent release
CC @sytses @markpundsack @eliran.mesika @bjung @dzaporozhets @axil @dhavens @pritianka