feat: support for textfile metrics (#13)

Add support for collecting arbitrary metrics from a textfile as well as
metrics collected from fail2ban. This allows other data to be exported
along with the fail2ban metrics (e.g. instance metadata).
Update the docker image to allow mounting a folder with a collection of
metric files to be exported. Only files ending in `.prom` with be read.
Update project README with the new functionality.

docker/run.sh

@@ -5,6 +5,8 @@
 
 db_path=/app/fail2ban.sqlite3
 socket_path=/var/run/fail2ban/fail2ban.sock
+textfile_dir=/app/textfile/
+textfile_enabled=false
 
 # Blank out the file paths if they do not exist - a hacky way to only use these files if they were mounted into the container.
 if [ ! -f "$db_path" ]; then
@@ -13,9 +15,14 @@ fi
 if [ ! -S "$socket_path" ]; then
     socket_path=""
 fi
+if [ -d $textfile_dir ]; then
+    textfile_enabled=true
+fi
 
 # Start the exporter (use exec to support graceful shutdown)
 # Inspired by: https://akomljen.com/stopping-docker-containers-gracefully/
 exec /app/fail2ban-prometheus-exporter \
     -db "$db_path" \
-    -socket "$socket_path"
+    -socket "$socket_path" \
+    -collector.textfile=$textfile_enabled \
+    -collector.textfile.directory="$textfile_dir"