@Test void merge_error() {
    List<DependencyLink> links = List.of(
      DependencyLink.newBuilder().parent("client").child("server").callCount(2L).build(),
      DependencyLink.newBuilder().parent("client").child("server").callCount(2L).build(),
      DependencyLink.newBuilder().parent("client").child("client").callCount(1L).build()
    );

    assertThat(DependencyLinker.merge(links)).containsExactly(
      DependencyLink.newBuilder().parent("client").child("server").callCount(4L).build(),
      DependencyLink.newBuilder().parent("client").child("client").callCount(1L).build()
    );
  }
Creates a link when there's a span missing, in this case 2L which is an RPC from web to app
zipkin/src/test/java/zipkin2/internal/DependencyLinkerTest.java