kube-prometheus/manifests/prometheus/prometheus-k8s-service-monitor-alertmanager.yaml

apiVersion: monitoring.coreos.com/v1alpha1
kind: ServiceMonitor
metadata:
  name: alertmanager
  labels:
    app: alertmanager
spec:
  selector:
    matchLabels:
      alertmanager: main
  namespaceSelector:
    matchNames:
    - monitoring
  endpoints:
  - port: web
    interval: 30s