code: public static Delay exponential(TimeUnit unit, long upper, long lower) {
          return exponential(unit, upper, lower, 1);
      }
docs: Creates a new {@link ExponentialDelay} with custom boundaries and default factor (eg. with upper 8, lower 3: 3,
  3, 4, 8, 8, 8...).
  
  @param unit the unit of the delay.
  @param upper the upper boundary.
  @param lower the lower boundary.
  @return a created {@link ExponentialDelay}.