fix: use targetPort instead of port on ServiceMonitor

[tomasbedrich]

What does this PR do?

fixes #852

More

• Yes, I updated the tests accordingly
• Yes, I ran make test and all the tests passed

[darkweaver87]

LGTM

[mloiseleur]

Thanks @tomasbedrich, for both clear bug report and this PR !

[jnoordsij]

Just noticed this change when upgrading to the new chart version. I think it doesn't do much wrong, but it is a behavioral change: the port options refers to the (Kubernetes) service port, while targetPort refers to the service pods. In this chart, both are called metrics, so no difference/issues there.

The reason why using the service monitor on its own does not work, is because the default service does not expose the metrics endpoint. So to have the service monitor working, you need to either add ports.metrics.expose = true to expose it, OR add a separate metrics service by adding metrics.prometheus.service.enabled = true (see #727).

So if issues arise with the new targetPort, this might be the solution to the original issue without changing the chart.