Investigate/Add alerts on availability of Error Tracking

Now we've our metrics available via Gitlab Ops Thanos(https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16395), we should start adding alerts to monitor Error Tracking availability.

After our open beta rollout, it would be helpful for us to be notified in case of errors.

See gitlab-com/runbooks!5022 (merged) that adds basic runbook and next iteration would be to add alerts and dashboards.

cc @mappelman @nicholasklick

Edited by Arun Sori