alerting-rules: ContinuousTaskEnded alert should not drop all unspecified labels (#8369)

substrate/.maintain/monitoring/alerting-rules/alerting-rules.yaml

@@ -135,7 +135,7 @@ groups:
 
   - alert: ContinuousTaskEnded
     expr: '(polkadot_tasks_spawned_total{task_name != "basic-authorship-proposer"} == 1)
-        - on(instance, task_name) (polkadot_tasks_ended_total == 1)'
+        - on(instance, task_name) group_left() (polkadot_tasks_ended_total == 1)'
     for: 5m
     labels:
       severity: warning