@Test void storeSpansCallback_onErrorWithMessage() {
    IllegalArgumentException error = new IllegalArgumentException("no beer");
    Callback<Void> callback = collector.new StoreSpans(TRACE);
    callback.onError(error);

    assertDebugLogIs("Cannot store spans [1, 1, 2, ...] due to IllegalArgumentException(no beer)");
    verify(metrics).incrementSpansDropped(4);
  }
Tags in zipkin v2 model are stringly typed.
zipkin-collector/core/src/test/java/zipkin2/collector/CollectorTest.java