tag 是用來提供 struct 中更多資訊的，只能在reflection中取得

What are the use(s) for tags in Go?

Tags in Golang

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. The tags are made visible through a reflection interface but are otherwise ignored.

// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
  microsec  uint64 "field 1"
  serverIP6 uint64 "field 2"
  process   string "field 3"
}

GopherCon 2015 had a presentation about struct tags called:

The Many Faces of Struct Tags (slide) (and a video)

Here is a list of commonly used tag keys: