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.
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.
Fabian Reinartz