prometheus: Implement basic Prometheus support

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

test/shell/t9.sh

@@ -0,0 +1,16 @@
+#!/bin/bash -e
+
+# run empty graph, with prometheus support
+timeout --kill-after=20s 15s ./mgmt run --tmp-prefix --prometheus &
+pid=$!
+sleep 5s	# let it converge
+
+# Check that etcd metrics are loaded
+curl 127.0.0.1:9233/metrics | grep "^etcd_server_has_leader 1"
+
+# Check that go metrics are loaded
+curl 127.0.0.1:9233/metrics | grep "^go_goroutines [0-9]\+"
+
+killall -SIGINT mgmt	# send ^C to exit mgmt
+wait $pid	# get exit status
+exit $?