code: public static double length(@NonNull Polygon polygon,
                                @NonNull @TurfConstants.TurfUnitCriteria String units) {
      double len = 0;
      for (List<Point> points : polygon.coordinates()) {
        len += length(points, units);
      }
      return len;
    }
docs: Takes a {@link Polygon} and measures its perimeter in the specified units. if the polygon
  contains holes, the perimeter will also be included.
  
  @param polygon geometry to measure
  @param units   one of the units found inside {@link TurfConstants.TurfUnitCriteria}
  @return total perimeter of the input polygon in the units specified
  @see <a href="http://turfjs.org/docs/#linedistance">Turf Line Distance documentation</a>
  @since 1.2.0