@Test void handleDecodeError_onErrorWithNullMessage() {
    RuntimeException error = new RuntimeException();
    collector.handleDecodeError(error, callback);

    verify(callback).onError(error);
    assertDebugLogIs("Cannot decode spans due to RuntimeException()");
    verify(metrics).incrementMessagesDropped();
  }
Tags in zipkin v2 model are stringly typed.
zipkin-collector/core/src/test/java/zipkin2/collector/CollectorTest.java