Alerting: Truncate PagerDuty summary when greater than 1024 characters (#18730)
Alerting: Truncate PagerDuty summary when greater than 1024 characters (#18730)

Requests to PagerDuty fail with an HTTP 400 if the `summary` 
attribute contains more than 1024 characters, this fixes this.
API spec:
https://v2.developer.pagerduty.com/docs/send-an-event-events-api-v2

Fixes #18727
grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana