TraceContext withExtra(List<Object> extra) {
    return new TraceContext(flags, traceIdHigh, traceId, localRootId, parentId, spanId, extra);
  }
@Test void withExtra_notEmpty() {
    assertThat(context.withExtra(Arrays.asList(1L)))
      .extracting("extraList")
      .isEqualTo(Arrays.asList(1L));
  }