@Test void lateParentIdInSharedSpan() {
    List<Span> withLateParent = new ArrayList<>(TRACE);
    withLateParent.set(2, TRACE.get(2).toBuilder().parentId(null).build());

    assertThat(new DependencyLinker().putTrace(withLateParent).link()).containsExactly(
      DependencyLink.newBuilder().parent("web").child("app").callCount(1L).build(),
      DependencyLink.newBuilder().parent("app").child("db").callCount(1L).errorCount(1L).build()
    );
  }
This test shows that if a parent ID is stored late (ex because it wasn't propagated), the span
can resolve once it is.
zipkin/src/test/java/zipkin2/internal/DependencyLinkerTest.java