-// LogRequests(logger, ...), and vice versa.
-func Instrument(logger *logrus.Logger, next http.Handler) Handler {
+// LogRequests(...), and vice versa.
+//
+// If registry is nil, a new registry is created.
+//
+// If logger is nil, logrus.StandardLogger() is used.
+func Instrument(registry *prometheus.Registry, logger *logrus.Logger, next http.Handler) Handler {