Go で不明な構造体を含む JSON 出力を拡張するにはどうすればよいですか?

func printInterface(val interface{}) {
    // Create a new struct type with anonymous field for the interface
    t2 := reflect.StructOf([]reflect.StructField{
        {Name: "X", Anonymous: true, Type: reflect.TypeOf(val)},
        {Name: "Extra", Type: reflect.TypeOf("")},
    })

    // Create a new value of the dynamic type
    v2 := reflect.New(t2).Elem()
    // Set the value of the anonymous field to the input interface
    v2.Field(0).Set(reflect.ValueOf(val))
    // Set the extra field to the desired value
    v2.FieldByName("Extra").SetString("text")

    json.NewEncoder(os.Stdout).Encode(v2.Interface())
}

func printInterface(val interface{}) error {
    // Marshal the value to JSON
    data, err := json.Marshal(val)
    if err != nil {
        return err
    }

    // Unmarshal the JSON into a map
    v2 := map[string]interface{}{}
    if err := json.Unmarshal(data, &v2); err != nil {
        return err
    }

    // Add the extra field
    v2["Extra"] = "text"

    // Marshal the map to JSON
    return json.NewEncoder(os.Stdout).Encode(v2)
}