google-auth-library-nodejs: Default User-Agent header is overwritten by DefaultTransporter

test-user-agent.ts

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/',
});

workaround.ts

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

const googleAuth = new GoogleAuth();

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

export class CustomTransporter extends DefaultTransporter {
  configure(opts?: GaxiosOptions | undefined): GaxiosOptions {
    const options = super.configure(opts);
    // If we have a default then overwrite the value.
    if (this.defaults.headers?.['User-Agent']) {
      if (!options.headers) {
        options.headers = {};
      }
      options.headers['User-Agent'] = this.defaults.headers['User-Agent'];
    }
    return options;
  }
}

idTokenClient.transporter = new CustomTransporter();

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

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