@Test void builder_merge_flags() {
    assertThat(Span.newBuilder().shared(true).merge(base.toBuilder().debug(true).build()).build())
      .isEqualToComparingFieldByField(base.toBuilder().shared(true).debug(true).build());
  }
Catches common error when zero is passed instead of null for a timestamp
zipkin/src/test/java/zipkin2/SpanTest.java