fix golint issues in rest (#529)

rest/handler/metrichandler.go

@@ -7,6 +7,7 @@ import (
 	"github.com/tal-tech/go-zero/core/timex"
 )
 
+// MetricHandler returns a middleware that stat the metrics.
 func MetricHandler(metrics *stat.Metrics) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {