在Go语言中，我们可以给结构体的字段加一个标签（tag），Go Language Specification 中有一段简短的描述：
A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.