code: public static Delay exponential(TimeUnit unit, long upper, long lower, long growBy, int powersOf) {
          return new ExponentialDelay(unit, upper, lower, growBy, powersOf);
      }
docs: Creates a new {@link ExponentialDelay} on a base different from powers of two, with custom boundaries and factor
  (eg. with upper 9000, lower 0, growBy 3, powerOf 10: 3, 30, 300, 3000, 9000, 9000, 9000, ...).
  
  @param unit the unit of the delay.
  @param upper the upper boundary.
  @param lower the lower boundary.
  @param growBy the multiplication factor (or basis for the size of each delay).
  @param powersOf the base for exponential growth (eg. powers of 2, powers of 10, etc...)
  @return a created {@link ExponentialDelay}.