Support for keeping @metadata in output plugins

We need to keep some @metadata fields across different instances of logstashes. In architecture "logstash -> kafka -> indexing logstash" the indexing logstash needs to know index names and some other values which are "computed" in the first logstash and stored in @metadata. Currently we are copying some @metadata fields to @anotherfield and use this field in indexing logstash. But it would be probably much better to have following option for all logstash output plugins:

"keep_metadata" => true/false (default false)

So we would be able to share @metadata.