Clusterlabel support for Kubernetes alerts

[hoangphuocbk]

Currently, when a Kubernetes alert is fired, for example KubePodCrashLooping, Clusterlabel is not mentioned in description
https://github.com/kubernetes-monitoring/kubernetes-mixin/blob/master/alerts/apps_alerts.libsonnet#L22

            annotations: {
              description: 'Pod {{ $labels.namespace }}/{{ $labels.pod }} ({{ $labels.container }}) is in waiting state (reason: "CrashLoopBackOff").',
              summary: 'Pod is crash looping.',
            },
            'for': '15m',
            alert: 'KubePodCrashLooping',

IMO, we should add clusterLabel to know which Kubernetes cluster the alert is belonged to.

 description: 'Pod {{ $labels.clusterLabel }}/{{ $labels.namespace }}/{{ $labels.pod }} ({{ $labels.container }}) is in waiting state (reason: "CrashLoopBackOff").'

[github-actions]

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