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*().