code: public int count(DynamoDBQueryExpression<T> queryExpression) {
          return mapper.<T>count(model.targetType(), queryExpression);
      }
docs: Evaluates the specified query expression and returns the count of matching
  items, without returning any of the actual item data
  @param queryExpression The query expression.
  @return The count.
  @see com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper#count