code: public static double lengthToRadians(double distance, @NonNull @TurfUnitCriteria String units) {
      return distance / FACTORS.get(units);
    }
docs: Convert a distance measurement (assuming a spherical Earth) from a real-world unit into
  radians.
  
  @param distance double representing a distance value
  @param units    pass in one of the units defined in {@link TurfUnitCriteria}
  @return converted distance to radians value
  @since 1.2.0