vectors

Is your feature request related to a problem? Please describe.
There are cases, when it might be useful to know how much points are there in index, which satisfies some filtering condition.
Currently, we can only estimate a total amount of points.

Describe the solution you'd like
Create an API which returns amount of points (estimated or precise, depending on flag) for a given filter

使用的3.2.7版本，3台服务器(1个master、3个ps、3个router)，按天建表(如：db_20210601、db_20210602、db_20210603)，每张表的结构如下 ：
`{
"name": "space",
"partition_num": 3,
"replica_num": 1,
"engine": {
"name": "gamma",
"index_size": 100000,
"retrieval_type": "IVFPQ",
"retrieval_param": {
"ncentroids": 2048,
"nsubvector": 64,
"metric_type": "InnerProduct"
},
"id_type": "String"
},
"propertie