String accessToken() {
        String urlString = String.format("%s/computeMetadata/v1/instance/service-accounts/default/token", endpoint);
        String accessTokenResponse = callGet(urlString);
        return extractAccessToken(accessTokenResponse);
    }
@Test
    public void accessToken() {
        // given
        stubFor(get(urlEqualTo("/computeMetadata/v1/instance/service-accounts/default/token"))
                .withHeader("Metadata-Flavor", equalTo("Google"))
                .willReturn(aResponse().withStatus(HttpURLConnection.HTTP_OK).withBody(accessTokenResponse(ACCESS_TOKEN))));

        // when
        String result = gcpMetadataApi.accessToken();

        // then
        assertEquals(ACCESS_TOKEN, result);
    }