Include position of same SD mechanisms within the same scrape configuration.
Move unique prefixing out of SD implementations and target manager into
its own interface.
In setups where the ServiceAddress is the relevant address for
scraping, users can relabel the `__address__` label to ServiceAddress
+ ":" + ServicePort.
This needs to be documented, of course. Will do once this is LGTM'd.
This change is conceptually very simple, although the diff is large. It
switches logging from "github.com/golang/glog" to
"github.com/prometheus/log", while not actually changing any log
messages. V(1)-style logging has been changed to be log.Debug*().