@Test void builder_merge_tags_concat() {
    Span merged = Span.newBuilder().putTag("1", "a").putTag("2", "a")
      .merge(base.toBuilder().putTag("2", "a").putTag("3", "a").build()).build();

    assertThat(merged).isEqualToComparingFieldByField(
      base.toBuilder().putTag("1", "a").putTag("2", "a").putTag("3", "a").build()
    );
  }
Catches common error when zero is passed instead of null for a timestamp
zipkin/src/test/java/zipkin2/SpanTest.java