You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: impersonate.go
    path of file: ./repos/moby/vendor/golang.org/x/oauth2/google/internal/externalaccount
    
    the code of the file until where you have to start completion: // Copyright 2021 The Go Authors. All rights reserved.
// Use of th
    
func (its ImpersonateTokenSource) Token() (*oauth2.Token, error) {
	lifetimeString := "3600s"
	if its.TokenLifetimeSeconds != 0 {
		lifetimeString = fmt.Sprintf("%ds", its.TokenLifetimeSeconds)
	}
	reqBody := generateAccessTokenReq{
		Lifetime:  lifetimeString,
		Scope:     its.Scopes,
		Delegates: its.Delegates,
	}
	b, err := json.Marshal(reqBody)
	if err != nil {
		return nil, fmt.Errorf("oauth2/google: unable to marshal request: %v", err)
	}
	client := oauth2.NewClient(its.Ctx, its.Ts)
	req, err := http.NewRequest("POST", its.URL, bytes.NewReader(b))
	if err != nil {
		return nil, fmt.Errorf("oauth2/google: unable to create impersonation request: %v", err)
	}
	req = req.WithContext(its.Ctx)
	req.Header.Set("Content-Type", "application/json")

	resp, err := client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("oauth2/google: unable to generate access token: %v", err)
	}
	defer resp.Body.Close()
	body, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return nil, fmt.Errorf("oauth2/google: unable to read body: %v", err)
	}
	if c := resp.StatusCode; c < 200 || c > 299 {
		return nil, fmt.Errorf("oauth2/google: status code %d: %s", c, body)
	}

	var accessTokenResp impersonateTokenResponse
	if err := json.Unmarshal(body, &accessTokenResp); err != nil {
		return nil, fmt.Errorf("oauth2/google: unable to parse response: %v", err)
	}
	expiry, err := time.Parse(time.RFC3339, accessTokenResp.ExpireTime)
	if err != nil {
		return nil, fmt.Errorf("oauth2/google: unable to parse expiry: %v", err)
	}
	return &oauth2.Token{
		AccessToken: accessTokenResp.AccessToken,
		Expiry:      expiry,
		TokenType:   "Bearer",
	}, nil
}