We've confirmed that all systems are back to normal with no customer impact as of 01/18, 11:00 UTC. Our logs show the incident started on 01/18, 10:00 UTC and that during the 1 hour that it took to resolve the issue some customers may have experienced missing or misfired alerts in East US region while using Azure Metric Alert Rules.
Root Cause: The failure was due to a recent deployment task
Incident Timeline: 1Hour & 0 minutes - 01/18, 10:00 UTC through 01/18, 11:00 UTC
We understand that customers rely on Metric Alerts as a critical service and apologize for any impact this incident caused.