Two stage build to reduce image size and build improvements provided by @imrehg.

Add (optional) metering support for use with Prometheus.

start.sh.metering

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+# Start the node exporter
+mkdir /mnt/ramdisk
+mount -t tmpfs -o size=8m tmpfs /mnt/ramdisk
+cd /mnt/ramdisk
+/opt/ttn-gateway/run.py &
+
+# WORKAROUND: Add symlink, as gwexporter otherwise currently don't seem to find this file
+ln -s /mnt/ramdisk/loragwstat.json /opt/gwexporter/loragwstat.json
+
+export PATH=$PATH:/opt/gwexporter/bin
+node /opt/gwexporter/gwexporter.js &
+
+cd /etc && ./node_exporter -web.listen-address ":81"