chore: Use http constants to replace numbers as parameters

Signed-off-by: xin.li <xin.li@daocloud.io>

server/etcdserver/api/rafthttp/stream.go

@@ -574,7 +574,7 @@ func (cr *streamReader) dial(t streamType) (io.ReadCloser, error) {
 			zap.String("address", uu.String()),
 		)
 	}
-	req, err := http.NewRequest("GET", uu.String(), nil)
+	req, err := http.NewRequest(http.MethodGet, uu.String(), nil)
 	if err != nil {
 		cr.picker.unreachable(u)
 		return nil, fmt.Errorf("failed to make http request to %v (%v)", u, err)