static CounterResult fromJson(String json) {
    return JsonUtil.parse(json, CounterResultParser::fromJson);
  }
@Test
  public void extraFields() {
    assertThat(
            CounterResultParser.fromJson("{\"unit\":\"bytes\",\"value\":23,\"extra\": \"value\"}"))
        .isEqualTo(CounterResult.of(Unit.BYTES, 23L));
  }