Template Source field in pagerduty payload

As a sane default we link to alertmanager, but
leave templating available to the user if
something suits their system better.

notify/impl.go

@@ -501,15 +501,12 @@ func (n *PagerDuty) notifyV2(ctx context.Context, eventType, key string, tmpl fu
 	if eventType == pagerDutyEventTrigger {
 		payload = &pagerDutyPayload{
 			Summary:       tmpl(n.conf.Description),
+			Source:        tmpl(n.conf.Client),
 			Severity:      n.conf.Severity,
 			CustomDetails: details,
 			Component:     n.conf.Component,
 			Group:         n.conf.Group,
 		}
-
-		if len(as) > 0 {
-			payload.Source = as[0].GeneratorURL
-		}
 	}
 
 	msg := &pagerDutyMessage{