prometheus stuff (#243)

Move some stuff around, fixup some readme.

And hopefully fix this:
sdr-enthusiasts/docker-adsb-ultrafeeder#160
(without needing to do hacks in the prometheus config)

README.md

@@ -756,7 +756,6 @@ In order for Telegraf to serve a [Prometheus](https://prometheus.io) endpoint, t
 | Variable            | Description                                                              |
 | ------------------- | ------------------------------------------------------------------------ |
 | `PROMETHEUS_ENABLE` | Set to `true` for a Prometheus endpoint on `http://0.0.0.0:9273/metrics` |
-| `PROMETHEUSPORT`    | TCP port for the Prometheus endpoint. Default value is 9273              |
 
 ## Minimalist setup
 

rootfs/etc/s6-overlay/startup.d/99-prometheus-conf

@@ -0,0 +1,27 @@
+#!/command/with-contenv bash
+#shellcheck shell=bash disable=SC1091
+
+source /scripts/common
+
+if ! chk_enabled "$PROMETHEUS_ENABLE"; then
+    exit 0
+fi
+
+# serve port 9274 always replying with stats.prom regardless of requested path
+
+cat > /etc/nginx/sites-enabled/prometheus_9274 <<EOF
+server {
+    listen 9274 default_server;
+    root /var/www/html;
+    server_name _;
+    location /metrics {
+        default_type text/plain;
+        alias /run/readsb/stats.prom;
+    }
+    location / {
+        default_type text/plain;
+        alias /run/readsb/stats.prom;
+        rewrite (.*) /metrics last;
+    }
+}
+EOF