code: func newAuthTokenFileRoundTripper(tokenFile string, rt http.RoundTripper) (http.RoundTripper, error) {
  	// fail-fast if we can't read the file.
  	_, err := ioutil.ReadFile(tokenFile)
  	if err != nil {
  		return nil, errors.Wrapf(err, "unable to read auth token file %s", tokenFile)
  	}
  	return &authTokenFileRoundTripper{tokenFile, rt}, nil
  }
docs: // newAuthTokenFileRoundTripper adds the auth token read from the file to a request.