static Span span(String traceId, @Nullable String parentId, String id, @Nullable Kind kind,
    @Nullable String local, @Nullable String remote, boolean isError) {
    Span.Builder result = Span.newBuilder().traceId(traceId).parentId(parentId).id(id).kind(kind);
    if (local != null) result.localEndpoint(Endpoint.newBuilder().serviceName(local).build());
    if (remote != null) result.remoteEndpoint(Endpoint.newBuilder().serviceName(remote).build());
    if (isError) result.putTag("error", "");
    return result.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