Fix two bugs introduced last commit.  First, Prometheus wasn't being killed because we were exiting with log.Fatalf.  Second, if the Prometheus listen address was given as :portnum (i.e. no host component), the target scrape config would match, and thus the instance labels of the resulting metrics.  This was undesirable from a dashboarding perspective.
Fix two bugs introduced last commit.  First, Prometheus wasn't being killed because we were exiting with log.Fatalf.  Second, if the Prometheus listen address was given as :portnum (i.e. no host component), the target scrape config would match, and thus the instance labels of the resulting metrics.  This was undesirable from a dashboarding perspective.

ncabatoff/prombench