Alerting: Firing alerts lost on rule delete

User impact:

Firing alerts lost

Steps to reproduce:

Go to alert rules tab and create some alerts using Percona templated alert (PostgreSQL too many connections for example) in same folder
Wait for firing state
Delete one of the alert rules
Check Firing alerts tab

Actual result:

All existing firing alerts are gone - back in normal/pending status again

Expected result:

Deleting an alert rule should not reset state of existing rules

Workaround:

Details: