repo-updater: Add critical alerts

Created by: ryanslade

The number of user added repos is greater than 90% of our hard limit of 200k.

Number of external services added is great than 20k

No external services have synced in more than 8 hours. By default we backoff for up to 8 hours, so anything higher than this indicates a problem.

The repo updater queue grows past 1000.

Closes: https://github.com/sourcegraph/sourcegraph/issues/14442