@Test void worksWhenMissingParentSpan() {
    String missingParentId = "a";
    List<Span> trace = List.of(
      span("a", missingParentId, "b", Kind.SERVER, "backend", "1.2.3.4", false),
      span("a", missingParentId, "c", Kind.SERVER, "backend", null, false)
    );

    assertThat(Trace.merge(trace)).containsExactlyElementsOf(trace);
  }
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