When the test ends, all files matching the watcher's glob are removed
via defer. In that moment, the draining goroutine may still be running
and then detect no files matching the configured glob just before the
test exits.
This is now solved by waiting for the draining goroutine to finish
before leaving the test function and thus causing the deferred file
removal.
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.
