initial prometheus metrics support, simplfied some code

internal/metrics/http_handler.go

@@ -0,0 +1,13 @@
+package metrics
+
+import (
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+func NewHandler() http.Handler {
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", promhttp.Handler())
+	return mux
+}