add simple prom file generation script

this is then ingested by docker-telegraf-adsb to provide autogain
statistics

Author: wiedehopf

rootfs/etc/s6-overlay/s6-rc.d/generate-prom/run

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec /scripts/generate-prom.sh

rootfs/etc/s6-overlay/s6-rc.d/generate-prom/type

@@ -0,0 +1 @@
+longrun

rootfs/etc/s6-overlay/s6-rc.d/user/contents.d/generate-prom

@@ -0,0 +1,25 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash
+
+# generate prom file for consumption by for example docker-telegraf-adsb
+# currently this only has autogain as the other metrics can be generated in docker-telegraf-adsb
+
+OUT=/run/skyaware978/stats.prom
+TMP=/run/skyaware978/stats.prom.tmp
+
+function generate() {
+    if [[ -f "$AUTOGAIN_CURRENT_VALUE_FILE" ]]; then
+        echo "autogain_current_value=$(cat "$AUTOGAIN_CURRENT_VALUE_FILE")"
+        echo "autogain_max_value=$(cat "$AUTOGAIN_MAX_GAIN_VALUE_FILE")"
+        echo "autogain_min_value=$(cat "$AUTOGAIN_MIN_GAIN_VALUE_FILE")"
+        echo "autogain_pct_strong_messages_max=$AUTOGAIN_PERCENT_STRONG_MESSAGES_MAX"
+        echo "autogain_pct_strong_messages_min=$AUTOGAIN_PERCENT_STRONG_MESSAGES_MIN"
+    fi
+}
+
+while sleep 60; do
+    generate > "$TMP"
+    mv -f "$TMP" "$OUT"
+done
+
+