Recently, prometheus-operator has been promoted to stable helm chart (<https://github.com/helm/charts/tree/master/stable/prometheus-operator>). 

I'd like to understand how to add a custom application to monitoring by prometheus-operator in a k8s cluster. An example for say gitlab runner which by default provides metrics on 9252 would be appreciated (<https://docs.gitlab.com/runner/monitoring/#configuration-of-the-metrics-http-server>).

I have a rudimentary yaml that obviously doesn't work but also not provides any feedback on *what* isn't working:

```
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: gitlab-monitor
  # Change this to the namespace the Prometheus instance is running in
  namespace: default
  labels:
    app: gitlab-runner-gitlab-runner
    release: prometheus
spec:
  selector:
    matchLabels:
      app: gitlab-runner-gitlab-runner
  namespaceSelector:
    # matchNames:
    # - default
    any: true
  endpoints:
  - port: http-metrics
    interval: 15s

```

This is the prometheus configuration:

```
> kubectl get prometheus -o yaml

...
serviceMonitorNamespaceSelector: {}
serviceMonitorSelector:
  matchLabels:
    release: prometheus
...

```

So the selectors should match. By "not working" I mean that the endpoints do not appear in the prometheus UI.