druid: KafkaEmitter for request emitting does not emit "queryType" for native queries

Affected Version

The Druid version where the problem was encountered: 0.22.x, 0.23.x

Description

When setting up a request emitter to use KafkaEmitter the queryType property is not serialized to a JSON. The problem is located in the method: public void emit(final Event event) of KafkaEmitter.java.

The problem is that the rendering of the JSON string is done on the Map<String, Object>. The Jackson ignores the polymorphic annotation on the Query class with queryType if the query is serialized as a part of the map (see: DefaultRequestLogEvent for explanation). If we serialize the query independently then the polymorphic annotation is respected.

Proposed solution

The simples solution is to serialize an object to JsonNode first. If the query is present, then serialize it independently and replace the non-polymorphic version in the JsonNode.