Warn on high description cardinality

[moshloop]

e.g. the following uses the message in the description (that maps to the endpoint prometheus label) causing cardinality explosion

apiVersion: canaries.flanksource.com/v1
kind: Canary
metadata:
  name: alertmanager-check
  labels:
    app: infra
spec:
  schedule: "@every 5m"
  alertmanager:
    - name: Prometheus
      icon: "prometheus"
      url: http://k8s-kube-prometheus-stack-alertmanager.monitoring.svc:9093/
      alerts:
        - .*
      ignore:
        - KubeScheduler.*
        - Watchdog
      transform:
        expr: |
          results.alerts.map(r,
             {
              'name': r.name,
              'labels': r.labels,
              'icon': 'prometheus',
              'message': r.message,
              'description': r.message,
            }
          ).toJSON()