.maintain/monitoring: Add alert when continuous task ends (#7250)

* .maintain/monitoring: Add alert when continuous task ends

Through the `polkadot_tasks_ended_total` Prometheus metric one can tell
when a task ended. Use this metric to alert when specific
known-to-be-continuous tasks end on a node.

* .maintain/monitoring: Don't hard-code task names

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

@@ -126,6 +126,16 @@ groups:
   # Others
   ##############################################################################
 
+  - alert: ContinuousTaskEnded
+    expr: '(polkadot_tasks_spawned_total == 1) - on(instance, task_name)
+    (polkadot_tasks_ended_total == 1)'
+    for: 5m
+    labels:
+      severity: warning
+    annotations:
+      message: 'Continuous task {{ $labels.task_name }} on node
+      {{ $labels.instance }} ended unexpectedly.'
+
   - alert: AuthorityDiscoveryDiscoveryFailureHigh
     expr: 'polkadot_authority_discovery_handle_value_found_event_failure /
     ignoring(name)