[ES|QL] export metadata fields as JSON

### Description

Can we scrape https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/MetadataAttribute.java#L43-L58 and publish it for Kibana? That way the autocomplete will always stay up-to-date.