code: public long getGap(MultiLongWatermark highWatermark) {
      Preconditions.checkNotNull(highWatermark);
      Preconditions.checkArgument(this.values.size() == highWatermark.values.size());
      long diff = 0;
      for (int i = 0; i < this.values.size(); i++) {
        Preconditions.checkArgument(this.values.get(i) <= highWatermark.values.get(i));
        diff += highWatermark.values.get(i) - this.values.get(i);
      }
      return diff;
    }
docs: Get the number of records that need to be pulled given the high watermark.