code: public static BackoffStrategy linearBackoff(long initialSleepTime, TimeUnit initialSleepTimeUnit,
                                                  long increment, TimeUnit incrementTimeUnit) {
          Preconditions.checkNotNull(initialSleepTimeUnit, "The initial wait time unit may not be null");
          Preconditions.checkNotNull(incrementTimeUnit, "The increment time unit may not be null");
          return new LinearBackoffStrategy(initialSleepTimeUnit.toMillis(initialSleepTime),
                  incrementTimeUnit.toMillis(increment));
      }
docs: Returns a strategy that waits a fixed amount of time after the first
  failed attempt and in incrementing amounts of time after each additional
  failed attempt.
  
  @param initialSleepTime     the time to wait before retrying the first time
  @param initialSleepTimeUnit the unit of the initial wait time
  @param increment            the increment added to the previous wait time after each failed attempt
  @param incrementTimeUnit    the unit of the increment
  @return a backoff strategy that incrementally waits an additional fixed time after each failed attempt