docs: public static BackoffStrategy join(BackoffStrategy... backoffStrategies) {
          Preconditions.checkState(backoffStrategies.length > 0, "Must have at least one backoff strategy");
          List<BackoffStrategy> waitStrategyList = Lists.newArrayList(backoffStrategies);
          Preconditions.checkState(!waitStrategyList.contains(null), "Cannot have a null backoff strategy");
          return new CompositeBackoffStrategy(waitStrategyList);
      }