@Test void toString_isJson() {
    assertThat(base.toString()).hasToString(
      "{\"traceId\":\"0000000000000001\",\"id\":\"0000000000000001\",\"localEndpoint\":{\"serviceName\":\"frontend\",\"ipv4\":\"127.0.0.1\"}}"
    );
  }
Catches common error when zero is passed instead of null for a timestamp
zipkin/src/test/java/zipkin2/SpanTest.java