Description: Fix prometheus http handler to use promhttp.
--- a/mongodb_exporter.go
+++ b/mongodb_exporter.go
@@ -14,6 +14,7 @@
 
 	"github.com/golang/glog"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
 func mongodbDefaultURI() string {
@@ -81,10 +82,10 @@
 }
 
 func prometheusHandler() http.Handler {
-	handler := prometheus.Handler()
+	handler := promhttp.Handler()
 	if hasUserAndPassword() {
 		handler = &basicAuthHandler{
-			handler:  prometheus.Handler().ServeHTTP,
+			handler:  promhttp.Handler().ServeHTTP,
 			user:     *authUserFlag,
 			password: *authPassFlag,
 		}
