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: urlcredsource.go
    path of file: ./repos/kubernetes/vendor/golang.org/x/oauth2/google/internal/externalaccount
    
    the code of the file until where you have to start completion: // Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package externalaccount

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"

	"golang.org/x/oauth2"
)

type urlCredentialSource struct {
	URL     string
	Headers map[string]string
	Format  format
	ctx     context.Context
}

func (cs urlCredentialSource) subjectToken() (string, error) {
	client := oauth2.NewClient(cs.ctx, nil)
	req, err := http.NewRequest("GET", cs.URL, nil)
	if err != nil {
		return "", fmt.Errorf("oauth2/google: HTTP request for URL-sourced credential failed: %v", err)
	}
	req = req.WithContext(cs.ctx)

	for key, val := range cs.Headers {
		req.Header.Add(key, val)
	}
	resp, err := client.Do(req)
	if err != nil {
		return "", fmt.Errorf("oauth2/google: invalid response when retrieving subject token: %v", err)
	}
	defer resp.Body.Close()

	respBody, err := ioutil.Read
    
func (cs urlCredentialSource) subjectToken() (string, error) {
	client := oauth2.NewClient(cs.ctx, nil)
	req, err := http.NewRequest("GET", cs.URL, nil)
	if err != nil {
		return "", fmt.Errorf("oauth2/google: HTTP request for URL-sourced credential failed: %v", err)
	}
	req = req.WithContext(cs.ctx)

	for key, val := range cs.Headers {
		req.Header.Add(key, val)
	}
	resp, err := client.Do(req)
	if err != nil {
		return "", fmt.Errorf("oauth2/google: invalid response when retrieving subject token: %v", err)
	}
	defer resp.Body.Close()

	respBody, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return "", fmt.Errorf("oauth2/google: invalid body in subject token URL query: %v", err)
	}
	if c := resp.StatusCode; c < 200 || c > 299 {
		return "", fmt.Errorf("oauth2/google: status code %d: %s", c, respBody)
	}

	switch cs.Format.Type {
	case "json":
		jsonData := make(map[string]interface{})
		err = json.Unmarshal(respBody, &jsonData)
		if err != nil {
			return "", fmt.Errorf("oauth2/google: failed to unmarshal subject token file: %v", err)
		}
		val, ok := jsonData[cs.Format.SubjectTokenFieldName]
		if !ok {
			return "", errors.New("oauth2/google: provided subject_token_field_name not found in credentials")
		}
		token, ok := val.(string)
		if !ok {
			return "", errors.New("oauth2/google: improperly formatted subject token")
		}
		return token, nil
	case "text":
		return string(respBody), nil
	case "":
		return string(respBody), nil
	default:
		return "", errors.New("oauth2/google: invalid credential_source file format type")
	}

}