@Test void deletesSelfReferencingParentId() {
    List<Span> trace = List.of(
      span("a", "a", "a", Kind.SERVER, "frontend", null, false),
      span("a", "a", "b", Kind.CLIENT, "frontend", null, false)
    );

    assertThat(Trace.merge(trace)).usingFieldByFieldElementComparator().containsExactlyInAnyOrder(
      span("a", null, "a", Kind.SERVER, "frontend", null, false),
      span("a", "a", "b", Kind.CLIENT, "frontend", null, false)
    );
  }
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