export build version as a prometheus label (#405)

This is how Prometheus recommends you do it, and how they do it
themselves in their client. This makes it easy to see which versions you
have deployed in your fleet, and query over it too.

main.go

@@ -396,7 +396,7 @@ func Main(config *Config, configTest bool, buildVersion string, logger *logrus.L
 		go lightHouse.LhUpdateWorker(ifce)
 	}
 
-	err = startStats(l, config, configTest)
+	err = startStats(l, config, buildVersion, configTest)
 	if err != nil {
 		return nil, NewContextualError("Failed to start stats emitter", nil, err)
 	}