Fix execution of exists query within nested queries on field with doc_values disabled (#78841) (#79462)

The FieldNamesFieldMapper adds non-indexed fields to a special metadata
field so that exists queries can be run efficiently.  This is built in a postParse
method that is run once per document.  However, this means that nested
documents are not handled correctly - non-indexed field names are added to
the parent document's metadata field rather than to the nested document's
field.

This commit fixes things to add non-indexed field names directly to the
nested documents.

Co-authored-by: Yoann Rodière <[email protected]>

Author: romseygeek