fixed calling registration multiple times

cmd/main.go

@@ -148,8 +148,18 @@ func main() {
 
 		go nw.watch(logger, watcher, cfgCont, ctx)
 
+		// register exporter only once
+		err = prometheus.Register(exporter)
+		if err != nil {
+			if !prometheus.Unregister(exporter) {
+				_ = level.Error(logger).Log("msg", "Exporter can't be unregistered")
+				return
+			}
+			prometheus.MustRegister(exporter)
+		}
+
 		metricHandlerFunc := collector.MetricHandler(exporter, *maxRequests, logger)
-		http.Handle(*metricsPath, promhttp.InstrumentMetricHandler(prometheus.DefaultRegisterer, utils.AddHSTSHeader(metricHandlerFunc)))
+		http.Handle(*metricsPath, utils.AddHSTSHeader(promhttp.InstrumentMetricHandler(prometheus.DefaultRegisterer, metricHandlerFunc)))
 		http.Handle("/-/ready", utils.AddHSTSHeader(readinessChecker()))
 		http.Handle("/-/healthy", utils.AddHSTSHeader(healthChecker()))
 

pkg/collector/exporter.go

@@ -102,15 +102,6 @@ func MetricHandler(exporter *Exporter, maxRequests int, logger log.Logger) http.
 		exporter.Mutex.RLock()
 		defer exporter.Mutex.RUnlock()
 
-		err := prometheus.Register(exporter)
-		if err != nil {
-			if !prometheus.Unregister(exporter) {
-				_ = level.Error(logger).Log("msg", "Exporter can't be unregistered")
-				return
-			}
-			prometheus.MustRegister(exporter)
-		}
-
 		// Delegate http serving to Prometheus client library, which will call collector.Collect.
 		handler := promhttp.HandlerFor(prometheus.DefaultGatherer, promhttp.HandlerOpts{
 			ErrorLog:            stdlog.New(log.NewStdlibAdapter(level.Debug(logger)), "prom_log: ", 0),

pkg/utils/utilities.go

@@ -60,7 +60,7 @@ func GetNamespace() string {
 	return "monitoring"
 }
 
-// HSTS header
+// add HSTS header
 func AddHSTSHeader(handler http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.Scheme == "https" {