This causes the Prometheus provider to take a stop channel as an
argument, which allows us to stop the lister (which keeps the series list
up to date) in the unit tests.
Several of the unit tests were out of date with the interface or type
definitions. Now that we're running CI, they need to be up to date
(they should have been anyway, but that's a different story).
k8s.io/custom-metrics-boilerplate was "renamed" to
"github.com/directxman12/custom-metrics-boilerplate". This updates all
references to point to the correct location.
The initial functionality works. There's still a number of TODOs to
clean up, and some edge cases to work around, and some errors that could
be handled better.
