Golang中奇怪的类型定义语法（名称，然后键入，然后是字符串文字）

I've been trying to find out how to use mgo (MongoDB driver for Go) and I came across this struct declaration:

type Something struct {
    Id bson.ObjectId "_id,omitempty"
    Name string
}

I don't quite understand the syntax of the first element (Id). I understand that it's being declared as type bson.ObjectId, but what is the string literal doing there?

My question is not about the mgo driver functionality,
but about this strange <name> <type> <string_literal> syntax.

I couldn't find anything on the Go specs, and I don't know how to google this either.

It's explained in the Struct types section of the language specification:

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"
}