ScrapeConfig is not pushed to PrometheusAgent objects

[stijzermans]

Is your feature request related to a problem? Please describe.
When using Prometheus in Agent mode, scrape configs are not pushed by the operator. The operator logs the following message:

main-controller.go:1546] error syncing 'infra-minio/minio': prometheuses.monitoring.coreos.com "prometheus-grafana-kube-pr-prometheus" not found

My prometheus is running with the following deployment.

apiVersion: monitoring.coreos.com/v1alpha1
kind: PrometheusAgent
metadata:
  name: prometheus-grafana-kube-pr-prometheus
  namespace: infra-monitoring
spec:
  ...

Operator is configured as following:

  values:
    operator:
      env:
        - name: PROMETHEUS_NAMESPACE
          value: "infra-monitoring"
        - name: PROMETHEUS_NAME
          value: "prometheus-grafana-kube-pr-prometheus"
        - name: WATCHED_NAMESPACE
          value: "infra-minio"

When running the operator with the same configuration, but Prometheus running as kind: Prometheus, it does work as expected.

Describe the solution you'd like
Both prometheuses.monitoring.coreos.com and prometheusagents.monitoring.coreos.com should be considered when searching for PROMETHEUS_NAME and pushing scraping config, not just prometheuses.monitoring.coreos.com

Describe alternatives you've considered
Using manual deployment of a probe (such as #2067) or a manual scrape config, but this removes the purpose of this functionality to dynamically provision this.

Can also mount the additionalScrapeConfigsSecret on PrometheusAgent, but that is be limited to a single scrape config secret.