feat: add listen address parameter

Add new -web.listen-address command line parameter, so that the
listening interface can be limited. This follows a similar style as
the official prometheus-node-exporter project.
Update project README with the new parameter.

README.md

@@ -33,6 +33,8 @@ See the [releases page](https://gitlab.com/hectorjsmith/fail2ban-prometheus-expo
 ```
 $ fail2ban-prometheus-exporter -h
 
+  -web.listen-address string
+        address to use for metrics server (default 0.0.0.0)
   -port int
         port to use for the metrics server (default 9191)
   -socket string

src/cfg/cfg.go

@@ -13,6 +13,7 @@ const (
 
 type AppSettings struct {
 	VersionMode          bool
+	MetricsAddress       string
 	MetricsPort          int
 	Fail2BanSocketPath   string
 	FileCollectorPath    string
@@ -22,6 +23,7 @@ type AppSettings struct {
 func Parse() *AppSettings {
 	appSettings := &AppSettings{}
 	flag.BoolVar(&appSettings.VersionMode, "version", false, "show version info and exit")
+	flag.StringVar(&appSettings.MetricsAddress, "web.listen-address", "0.0.0.0", "address to use for the metrics server")
 	flag.IntVar(&appSettings.MetricsPort, "port", 9191, "port to use for the metrics server")
 	flag.StringVar(&appSettings.Fail2BanSocketPath, "socket", "", "path to the fail2ban server socket")
 	flag.BoolVar(&appSettings.FileCollectorEnabled, "collector.textfile", false, "enable the textfile collector")

src/exporter.go

@@ -53,7 +53,7 @@ func main() {
 	if appSettings.VersionMode {
 		printAppVersion()
 	} else {
-		addr := fmt.Sprintf("0.0.0.0:%d", appSettings.MetricsPort)
+		addr := fmt.Sprintf("%s:%d", appSettings.MetricsAddress, appSettings.MetricsPort)
 
 		log.Printf("starting fail2ban exporter at %s", addr)