@Test void handleStorageError_onErrorWithNullMessage() {
    RuntimeException error = new RuntimeException();
    collector.handleStorageError(TRACE, error, callback);

    verify(callback).onError(error);
    assertDebugLogIs("Cannot store spans [1, 1, 2, ...] due to RuntimeException()");
    verify(metrics).incrementSpansDropped(4);
  }
Tags in zipkin v2 model are stringly typed.
zipkin-collector/core/src/test/java/zipkin2/collector/CollectorTest.java