Move prometheus-agent pre-requisites to new method to avoid creating controller incase of validation failure

 
During prometheus-operator bump to 0.64 in openshift fork which includes prometheus-agent CRD support e2e test is failing since it throws alert PrometheusOperatorNotReady  for prometheus-agent controller since prometheus-agent CRD is not installed. We don't want include this CRD at this time in OCP so we need to adjust the how the metrics is registered in upstream prometheus-operator code. We don't want to register the metric if prometheus-agent requisites are not met.

cc: spasquie@redhat.com