protected void writeToDlq(Event event, String errorMessage) {
        if (dlqWriter != null) {
            try {
                // because our event is co.elastic.logstash.api.Event,
                // we need to cast it to org.logstash.Event
                // to be able to use writeEntry
                org.logstash.Event castedEvent = (org.logstash.Event) event;
                dlqWriter.writeEntry(castedEvent, this.getName(), this.id, errorMessage);
            } catch (IOException e) {
                logger.error("Error writing to dead letter queue: ", e);
            }
        }
    }
We need to use the original event object to get the fieldName value.
For some reason, getting it from the eventData map of asyncFeed doesn't work

@param event The original event object
@param fieldName The field name to get
@return The value of the field or the field name if it doesn't exist
integration/logstash-plugins/logstash-output-vespa/src/main/java/org/logstashplugins/VespaFeed.java