feat: add support for basic auth (#16)

Add new CLI parameters to enable protecting the API endpoints with basic
auth authentication.
Wrap the server endpoints in a new auth middleware that protects it using
the provided basic auth credentials (if set).
Store the provided basic auth credentials as hashed values to prevent them
from being accidentally leaked.
Add unit tests to ensure the new functionality works as expected.

src/exporter.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fail2ban-prometheus-exporter/auth"
 	"fail2ban-prometheus-exporter/cfg"
 	"fail2ban-prometheus-exporter/collector/f2b"
 	"fail2ban-prometheus-exporter/collector/textfile"
@@ -63,10 +64,13 @@ func main() {
 		textFileCollector := textfile.NewCollector(appSettings)
 		prometheus.MustRegister(textFileCollector)
 
-		http.HandleFunc("/", rootHtmlHandler)
-		http.HandleFunc(metricsPath, func(w http.ResponseWriter, r *http.Request) {
-			metricHandler(w, r, textFileCollector)
-		})
+		http.HandleFunc("/", auth.BasicAuthMiddleware(rootHtmlHandler, appSettings.BasicAuthProvider))
+		http.HandleFunc(metricsPath, auth.BasicAuthMiddleware(
+			func(w http.ResponseWriter, r *http.Request) {
+				metricHandler(w, r, textFileCollector)
+			},
+			appSettings.BasicAuthProvider,
+		))
 		log.Printf("metrics available at '%s'", metricsPath)
 
 		svrErr := make(chan error)