Alerting: Support passing tags to Pagerduty and allow notification on specific event categories (#21335)

Add support for passing tags to Pagerduty and allow notification 
on specific event categories such as Class, Group and Component.

Ref #19912, #19913

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

docs/sources/alerting/notifications.md

@@ -103,6 +103,8 @@ Integration Key | Integration key for PagerDuty.
 Severity | Level for dynamic notifications, default is `critical`
 Auto resolve incidents | Resolve incidents in PagerDuty once the alert goes back to ok
 
+**Note:** The tags `Class`, `Group`, and `Component` have special meaning in the [Pagerduty Common Event Format - PD-CEF](https://support.pagerduty.com/docs/pd-cef).  If an alert panel defines these tag keys they will be transposed to the root of the event sent to Pagerduty.  This means they will be available within the Pagerduty UI and Filtering tools.
+
 ### Webhook
 
 The webhook notification is a simple way to send information about a state change over HTTP to a custom endpoint.
@@ -188,7 +190,7 @@ Kafka | `kafka` | yes, external only | no
 Line | `line` | yes, external only | no
 Microsoft Teams | `teams` | yes, external only | no
 OpsGenie | `opsgenie` | yes, external only | yes
-Pagerduty | `pagerduty` | yes, external only | no
+Pagerduty | `pagerduty` | yes, external only | yes
 Prometheus Alertmanager | `prometheus-alertmanager` | yes, external only | yes
 Pushover | `pushover` | yes | no
 Sensu | `sensu` | yes, external only | no