成员的完整列表，这些成员属于 EMChatMessageStatistics,包括所有继承而来的类成员

	attachmentSize	EMChatMessageStatistics
	chatType	EMChatMessageStatistics
	direction	EMChatMessageStatistics
	from	EMChatMessageStatistics
	messageId	EMChatMessageStatistics
	messageSize	EMChatMessageStatistics
	thumbnailSize	EMChatMessageStatistics
	timestamp	EMChatMessageStatistics
	to	EMChatMessageStatistics
	type	EMChatMessageStatistics