Alerting: Scheduler to drop ticks if a rule's evaluation is too slow (#48885)

* drop ticks if evaluation of a rule is too slow.
* add metric schedule_rule_evaluations_missed_total

pkg/services/ngalert/models/alert_rule.go

@@ -125,6 +125,7 @@ type AlertRule struct {
 }
 
 type SchedulableAlertRule struct {
+	Title           string
 	UID             string `xorm:"uid"`
 	OrgID           int64  `xorm:"org_id"`
 	IntervalSeconds int64