pod service discovery: use port names in k8s configs to select prometheus ports

[stephen-soltesz]

Sometimes we declare multiple ports in the k8s daemonset or deployment configuration. Our existing service discovery rules in prometheus tries to scrape all declared ports. This could be simplified by naming the ports in a specific way (e.g. "prometheus-port") so that the SD rules know to select only this port.

This would only be helpful if this could be applied to multiple ports for multiple containers with in a pod.