Update flux custom metrics examples/docs to support resource-state-metrics

The monitoring setup example located in this repository https://github.com/fluxcd/flux2-monitoring-example and the official documentation located here https://fluxcd.io/flux/monitoring/custom-metrics/ leverage the custom-resource-state feature in kube state metrics.

This feature is now feature frozen in favour of resource-state-metrics, and will be deprecated once resource-state-metrics is stable.

It would be great if the examples and docs were complemented with instructions about how to get flux custom metrics working with resource-state-metrics.