cr-audit-commits needs to alert in the following situations

 - when a ref's scanning has fallen behind
 - when a ref's auditing has fallen behind
 - when a ref's notifications have fallen behind.

Possible approaches to set up these alarms:
 The cron job handler needs to be returning 200's every 5 minutes, raise an alert if it hasn't run in one hour.
 The cron job increments RefAuditsDue metric, raise an alert if 3 or more refs failed to be scheduled in the last hour.
 We should raise an alert if for over an hour, we have scanned more auditable commits than we have actually audited (meaning some commits that need to be audited haven't)
 We should raise an alert if the error ratio > 0 is sustained over an hour.