Drupal 9: Migrating Textarea Fields Into Metatag Data

This will migrate text area field data into Metatag module fields.

process:
  metatags/0/description: field_meta_data/0/value
  metatags/0/title: field_meta_title/0/value
  field_meta_tags:
    plugin: callback
    callable: serialize
    source: "@metatags"