How to create a ServiceMonitor for prometheus-operator?

This image perfectly shows the connection between Prometheus,ServiceMonitors and Services

If any of the matches are not correct, targets won't show up.

Read more: https://github.com/prometheus-operator/prometheus-operator/blob/main/Documentation/troubleshooting.md#troubleshooting-servicemonitor-changes

I know this question is already answered. But I had a similar problem when Prometheus deployed in Kubernetes with Helm's stable/prometheus-operator chart couldn't find any active targets for my ServiceMonitor. It turned out that my Service exposed a port that I didn't explicitly named:

  - protocol: TCP
    port: 8080
    targetPort: uwsgi

I could use it in Ingress by targeting uwsgi port. But it seems that ServiceMonitor needs an explicitly named port in Service even if it has the same name as its own tagetPort:

  - name: uwsgi
    protocol: TCP
    port: 8080
    targetPort: uwsgi

I have written a blog post about this problem here

Thanks to Peter who showed me that it idea in principle wasn't entirely incorrect I've found the missing link. As a servicemonitor does monitor services (haha), I missed the part of creating a service which isn't part of the gitlab helm chart. Finally this yaml did the trick for me and the metrics appear in Prometheus:

# Service targeting gitlab instances
apiVersion: v1
kind: Service
metadata:
  name: gitlab-metrics
  labels:
    app: gitlab-runner-gitlab-runner
spec:
  ports:
  - name: metrics # expose metrics port
    port: 9252 # defined in gitlab chart
    targetPort: metrics
    protocol: TCP
  selector:
    app: gitlab-runner-gitlab-runner # target gitlab pods
---
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: gitlab-metrics-servicemonitor
  # 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 # target gitlab service
  endpoints:
  - port: metrics
    interval: 15s

Nice to know: the metrics targetPort is defined in the gitlab runner chart.