feat: collect new up metric from fail2ban socket

Add support for connecting the exporter directly to the fail2ban server's
socket to send requests and receive data. The path to the socket file is
optional and specified on startup.
Export a new metric based on the response of the `ping` command sent to the
fail2ban server. The metric is set to 1 if the server responds with `pong`
and 0 in any other case. This metric is only shown if the path to the
socket file was provided on startup.

src/cfg/cfg.go

@@ -12,9 +12,10 @@ const (
 )
 
 type AppSettings struct {
-	VersionMode    bool
-	MetricsPort    int
-	Fail2BanDbPath string
+	VersionMode        bool
+	MetricsPort        int
+	Fail2BanDbPath     string
+	Fail2BanSocketPath string
 }
 
 func Parse() *AppSettings {
@@ -22,6 +23,7 @@ func Parse() *AppSettings {
 	flag.BoolVar(&appSettings.VersionMode, "version", false, "show version info and exit")
 	flag.IntVar(&appSettings.MetricsPort, "port", 9191, "port to use for the metrics server")
 	flag.StringVar(&appSettings.Fail2BanDbPath, "db", "", "path to the fail2ban sqlite database")
+	flag.StringVar(&appSettings.Fail2BanSocketPath, "socket", "", "path to the fail2ban server socket")
 
 	flag.Parse()
 	appSettings.validateFlags()
@@ -31,8 +33,8 @@ func Parse() *AppSettings {
 func (settings *AppSettings) validateFlags() {
 	var flagsValid = true
 	if !settings.VersionMode {
-		if settings.Fail2BanDbPath == "" {
-			fmt.Println("missing flag 'db'")
+		if settings.Fail2BanDbPath == "" && settings.Fail2BanSocketPath == "" {
+			fmt.Println("at least one of the following flags must be provided: 'db', 'socket'")
 			flagsValid = false
 		}
 		if settings.MetricsPort < minServerPort || settings.MetricsPort > maxServerPort {