@Test void cleanupComparator_ordersClientFirst() {
    List<Span> trace = Arrays.asList( // to allow sorting
      span("a", "a", "b", Kind.SERVER, "backend", "1.2.3.5", false),
      span("a", "a", "b", Kind.CLIENT, "frontend", null, false)
    );

    Collections.sort(trace, Trace.CLEANUP_COMPARATOR);
    assertThat(trace.get(0).kind()).isEqualTo(Kind.CLIENT);
  }
If a client request is proxied by something that does transparent retried. It can be the case
that two servers share the same ID (accidentally!)
zipkin/src/test/java/zipkin2/internal/TraceTest.java