discovery/marathon/marathon.go
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
}
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
}
// newAuthTokenFileRoundTripper adds the auth token read from the file to a request.