Add MetricsBackend with prometheus counter support

util/ci/build_prometheus_cpp.sh

@@ -0,0 +1,13 @@
+#! /bin/bash -eu
+
+cd /tmp
+git clone --recursive https://github.com/jupp0r/prometheus-cpp
+mkdir prometheus-cpp/build
+cd prometheus-cpp/build
+cmake .. \
+	-DCMAKE_INSTALL_PREFIX=/usr/local \
+	-DCMAKE_BUILD_TYPE=Release \
+	-DENABLE_TESTING=0
+make -j2
+sudo make install
+