function getRectLength(el) {
  const w = el.getAttribute('width');
  const h = el.getAttribute('height');
  return (w * 2) + (h * 2);
}
Returns the `<rect>` length.
It doesn't compute `rx` and / or `ry` of the element.
@see http://stackoverflow.com/a/30376660
@param {SVGRectElement} el target element
@returns {number} the `<rect>` length