Add listing in ent client and add metrics

@soundcloud/iss @ohm  @juliusv 
It's quite common in Go to just pass the whole address like "localhost:1234" on the command line instead of separate host and port. Maybe make just one flag called `http.address`? Hmm, `listen.address`? `telemetry.address`? Maybe the latter is the clearest name if the only thing you do on this port is telemetry (sorry, I know this goes against what I said earlier, but somehow I thought there was another port open already doing other stuff... forgot that doozerd-backup doesn't have a normal port besides telemetry).