document的核心元数据
document的核心元数据有三个：_index、_type、_id
初始化数据：
PUT test_index/test_type/1 { "test_content":"test test" }
{ "_index": "test_index", "_type": "test_type", "_id": "1", "_version": 1, "result": "created", "_shards": { "total": 2, "successful": 1, "failed": 0 }, "created": true }