如何通过Golang获取gcp承载令牌

Question

1.I'm try to use google translation api V3beta1 to translate words, I use http post to send request, but how to get bearer token by golang? 2.Or if I use grpc protocol, whether can I don't pass the bearer token. and where can I get the v3beta1 grpc interface file.

I test translate v2 rest api, it's ok whitout bearer token,but it return 401 code when I use v3beta1 api, I notice there is a bearer token need to pass, but I don't how to get it with golang.

//code such here
headMap := &http.Header{}
    headMap.Add("Content-Type", "application/json")
    headMap.Add("Accept", "application/json")
    //headMap.Add("Authorization", "Bearer (how to get this bear)")
    bodyParams := goov3Req{
        Content: "你好",
        SourceLanguageCode:"zh",
        TargetLanguageCode:"en",
        //MimeType:"text",
        Model:"nmt",
        //Key:googleRestFulKey,
    }
    fmt.Println(bodyParams)

    str, _ := json.Marshal(bodyParams)

    var b []byte
    var e error

    _, b, _, e = util.DoPost("google_its", googleRestFulUrl, *headMap, []byte(str), timeout)

"error": {


----------

//get error code
    "code": 401,
    "message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
    "status": "UNAUTHENTICATED"
  }