I migrated nodes including metatags from D7 to D8. As soon as metatags are not field in D7 I do not have it as common source in d7_node plugin. Of course, I can extend this plugin with my own and make join with metatag database table. But this variant seems for me not ideal. As I understand I can't execute any db queries inside process plugins, so I need to feed my custom source plugin result to a metatag field in a process section of my migration config. How can I do it and is it possible?
Yes, it is possible to migrate fields but I don't know Metatags.There is an existing issue in Metatags for creating a migration, it may be of help. https://www.drupal.org/project/metatag/issues/2563649