import { AuthClient, IdTokenClient } from 'google-auth-library';

const googleAuth = new GoogleAuth();

const idTokenClient = await googleAuth.getIdTokenClient('https://www.example.com/');

idTokenClient.gaxios.defaults.headers = {
  'User-Agent': 'foo/1',
};

idTokenClient.request({
  method: 'GET',
  url: 'https://www.example.com/',
});