Metadata editor JS is /libs/dam/gui/coral/components/admin/metadataeditor/clientlibs/metadataeditor/js/metadataeditor.js.
Metadata is updated with the below request on click of save and close button

Adobe suggested way of adding more metadata is to
1. create metadata profile with the properties, (Metadata Profiles )
2. apply it on the asset folder,
3. update metadata schema and map the fields to the new fields (Metadata Schemas )