InfluxDB 的查询语句中必须用单引号，不能用双引号。这是一个 坑

错误的写法：

> SELECT * FROM cpu WHERE host="serverA"
ERR: unknown field or tag name in where clause: serverA

正确的写法：

> SELECT * FROM cpu WHERE host='serverA'
name: cpu
tags: host=serverA, region=us_west
time				value
----				-----
2015-08-24T16:23:50.537452457Z	0.64