Get the notification rule of one priority

The rule for one recipient on one priority. There is no identifier: the row is addressed by recipient and priority, and all four priorities are always present in a listing — an unconfigured one shows what would apply by default.

Where the priority of an observability alert comes from. Not from the alert's severity. A routing rule whose action is escalate produces a critical notification; every other notifying action produces a normal one. So a rule keyed on critical here is reached by alerts an escalate routing rule matched, whatever their severity was, and a warning alert routed with message lands on the normal rule. Configuring critical and expecting Grafana's severity: critical to arrive there is the mistake this note exists to prevent — see the AccumulationRule schema.