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For me the following snipet works with Longhorn:

apiVersion: <http://monitoring.coreos.com/v1|monitoring.coreos.com/v1>
kind: ServiceMonitor
metadata:
  name: longhorn-prometheus-servicemonitor
  namespace: longhorn-system
spec:
  endpoints:
  - port: manager
  namespaceSelector:
    matchNames:
    - longhorn-system
  selector:
    matchLabels:
      app: longhorn-manager

Maybe you should restart the prometheus after adding the servicemonitor.