We've confirmed that all systems are back to normal with no customer impact as of 07/31, 03:15 UTC. Our logs show the incident started on 07/31, 01:00 UTC and that during the 2 hours & 15 minutes that it took to resolve the issue some customers in West Europe may have not received some metric alert notifications.
Root Cause: The failure was due to one of back-end services became unhealthy, impacting query monitor performance, causing some of the metrics to be delayed. Correct metrics were not flowing for evaluation, preventing some alerts from being generated.
Incident Timeline: 2 Hours & 15 minutes - 07/31, 01:00 UTC through 07/31, 03:15 UTC
We understand that customers rely on Azure monitor as a critical service and apologize for any impact this incident caused.