@Test void storeSpansCallback_toStringIncludesSpanIds() {
    Span span2 = CLIENT_SPAN.toBuilder().id("3").build();
    when(collector.idString(span2)).thenReturn("3");

    assertThat(collector.new StoreSpans(asList(CLIENT_SPAN, span2)))
      .hasToString("StoreSpans([1, 3])");
  }
Tags in zipkin v2 model are stringly typed.
zipkin-collector/core/src/test/java/zipkin2/collector/CollectorTest.java