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*().
Some SD configs may have many options. To be readable and consistent, make
all discovery constructors receive the full config rather than the separate
arguments.
This commits adds file based service discovery which reads target
groups from specified files. It detects changes based on file watches
and regular refreshes.
