code: public static Delay linear(TimeUnit unit, long upper, long lower, long growBy) {
          return new LinearDelay(unit, upper, lower, growBy);
      }
docs: Creates a new {@link LinearDelay} with a custom boundaries and factor.
  
  @param unit the unit of the delay.
  @param upper the upper boundary.
  @param lower the lower boundary.
  @param growBy the multiplication factor.
  @return a created {@link LinearDelay}.