Add example on how to update prometheus targets

Updates a target list in the format expected by file_sd_config[0] every
20 seconds.

[0]: https://prometheus.io/docs/prometheus/latest/configuration/configuration/#file_sd_config

examples/updater/pushprox-updater.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Update the prometheus static file config based off of PushProx
+
+[Service]
+User=pushprox-updater
+Group=pushprox-updater
+
+EnvironmentFile=-/etc/default/pushprox-updater
+ExecStart=/usr/bin/pushprox-updater
+
+NoNewPrivileges=true
+PrivateTmp=yes
+PrivateDevices=true

examples/updater/pushprox-updater.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Update the prometheus static file config based off of PushProx
+
+[Timer]
+OnStartupSec=20s
+OnUnitInactiveSec=20s
+
+[Install]
+WantedBy=timers.target

examples/updater/update.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+set -euo pipefail
+
+host=${PUSHPROX_HOST:-localhost:8080}
+ports=${PUSHPROXY_PORTS:-9100}
+sd_file=${PUSHPROXY_FILE:-/etc/prometheus/file_sd/clients.json}
+
+tmpfile=$(mktemp --suffix "pushprox-target-updater")
+trap 'rm -f "$tmpfile"' EXIT
+
+if ! curl -s "$host/clients" | jq --argjson ports "[$ports]" '
+. | map(.targets[0] as $target | {
+        targets: $ports | map(tostring) | map($target + ":" + .)
+})' > "$tmpfile"; then
+    echo >&2 "Failed to update inventory"
+    exit 2
+fi
+
+chmod a+r "$tmpfile"
+mv "$tmpfile" "$sd_file"