copy message key from consumed to produced record

Signed-off-by: Sebastian Schüpbach <sebastian.schuepbach@unibas.ch>

src/main/kotlin/KafkaTopology.kt

@@ -111,7 +111,7 @@ class KafkaTopology(
                     }
 
                     var producerRecord: ProducerRecord<String, String> =
-                        ProducerRecord(outputTopic, event.eventId, modelString)
+                        ProducerRecord(outputTopic, it.key(), modelString)
                     if (headerData.containsKey("institutionId")) {
                         producerRecord.headers().add("institutionId", headerData["institutionId"]?.toByteArray())
                     }