Logstash Core Event Java vs Ruby have different behavior

I detected a bug on logstash-core-event-java.

We have Pure Java-libraries performing all the Filtering and we do a small "hack" to have @metadata inside the event:

`@some_filter.filter event.to_hash, event["@metadata"]`

And within the Java Filter object, we just put the metadata map inside the event so that we have a Map for everything

```
private static final String METADATA_FIELDREF = "@metadata";

@Override
public void filter(Map eventData, Map metaData) {
eventData.put(METADATA_FIELDREF, metaData);
doFilter(eventData);
eventData.remove(METADATA_FIELDREF);
}
```

This behaviour occurs only on logstash-core-event-java, and this got fixed by switching back to logstash-core-event library.