Alerting: Enable setting of OpsGenie priority via a tag (#21298)

OpsGenie's model works heavily off of the priority of an alert, 
e.g. routing and escalation. Currently this plugin only supports 
the default "P3".
Setting a tag og_priority to the correct P-value, e.g. P1, P2, P3, 
P4 or P5, will call the OpsGenie API with the correct priority value 
set.

docs/sources/administration/provisioning.md

@@ -430,6 +430,7 @@ The following sections detail the supported settings for each alert notification
 | apiKey |
 | apiUrl |
 | autoClose |
+| overridePriority |
 
 #### Alert notification `telegram`