foo.go

package main

import (
	"encoding/json"
	"fmt"
)

type Item struct {
	Raw    json.RawMessage
	Values map[string][]int
}

var _ json.Unmarshaler = new(Item)

func (i *Item) UnmarshalJSON(raw []byte) error {
	i.Raw = raw
	return json.Unmarshal(raw, &i.Values)
}

func main() {
	raw := []byte(`
{
	"foo": [1, 2, 3],
	"bar": [4, 5, 6]
}
`)
	var i Item
	if err := json.Unmarshal(raw, &i); err != nil {
		panic(err)
	}
	fmt.Printf("%s\n", i.Raw)
	fmt.Printf("%v\n", i.Values)
}