the alert `AlertmanagerClusterFailedToSendAlerts` is not triggered even though alerting is misconfigured 

OCP 4.20.4

Install an OCP 4.20.4 cluster, deliberately mis-configure alertconfig (in this case wrong mail server port) to ensure alerting is failing

    1. Install OCP 4.10.4
    2. mis-configure alerting, e.g. configure mail receiver with wrong mail server port to ensure all alertmanager instances fail to send alerts
    3. check for alerts
    

just showing alerts:
- Watchdog
- AlertmanagerFailedToSendAlerts

showing in addition to the actual results
- AlertmanagerClusterFailedToSendAlerts

 it seems the expression for the alert is wrint, in particular the bracket close to the end should be moved to after 0.01:


$ oc get PrometheusRule -o yaml alertmanager-main-rules  |grep -A 10 AlertmanagerClusterFailedToSendAlerts
    - alert: AlertmanagerClusterFailedToSendAlerts
      annotations:
        description: The minimum notification failure rate to {{ $labels.integration
          }} sent from any instance in the {{$labels.job}} cluster is {{ $value |
          humanizePercentage }}.
        runbook_url: https://github.com/openshift/runbooks/blob/master/alerts/cluster-monitoring-operator/AlertmanagerClusterFailedToSendAlerts.md
        summary: All Alertmanager instances in a cluster failed to send notifications
          to a critical integration.
      expr: |
        min by (namespace,service, integration) (
          rate(alertmanager_notifications_failed_total{job=~"alertmanager-main|alertmanager-user-workload", integration=~`.*`}[15m])
        /
          ignoring (reason) group_left rate(alertmanager_notifications_total{job=~"alertmanager-main|alertmanager-user-workload", integration=~`.*`}[15m])
        )                 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        > 0.01            <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
      for: 5m


changing it to

 [...]
          ignoring (reason) group_left rate(alertmanager_notifications_total{job=~"alertmanager-main|alertmanager-user-workload", integration=~`.*`}[15m])
        > 0.01            <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        ) <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

seems to work