Makefile: add checkmetrics target, use in CI (#797)

checkmetrics.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+
+if [[ ( -z "$1" ) || ( -z "$2" ) ]]; then
+    echo "usage: ./checkmetrics.sh /usr/bin/promtool e2e-output.txt"
+    exit 1
+fi
+
+# Only check node_exporter's metrics, as the Prometheus Go client currently
+# exposes a metric with a unit of microseconds.  Once that is fixed, remove
+# this filter.
+lint=$($1 check metrics < $2 2>&1 | grep "node_")
+
+if [[ ! -z $lint ]]; then
+    echo -e "Some Prometheus metrics do not follow best practices:\n"
+    echo "$lint"
+
+    exit 1
+fi