Use prometheus as a metric backend

This entails writing a metric aggregation program since each worker has its
own memory, and thus own metrics because of python gunicorn. The python
client is a simple wrapper that makes web requests to it.

local-docker.sh

@@ -30,6 +30,11 @@ initdb)
 fulldbtest)
 	d bash /src/quay/test/fulldbtest.sh
 	;;
+prom)
+	R=quay.io/quay/prom-monitor
+	docker build -t $R prom_aggregator
+	docker run --rm -it --net=host $R
+	;;
 *)
 	echo "unknown option"
 	exit 1