By default, elements in _source fields are accessible. How do I reference _source siblings in logstash output config? e.g. highlight, _id, _score, _index etc.
It is unclear from your post what you're intending to accomplish.
Typically, highlight and _score are attributes of search query response from Elasticsearch, indicating how your specific query got the the documents returned in the result set, while _id and _index are attributes of each document, indicating the document's id and the index in which it is stored.
As such, they aren't present or accessible in a Logstash output to Elasticsearch, because the document has not yet been persisted to Elasticsearch.