There is a dynamic hook right before any new metadata is added:
do_action( "add_{$meta_type}_meta", $object_id, $meta_key, $_meta_value );
It would be add_post_meta for posts accordingly.
Note that you might to tinker some to catch this precisely in the case of new post being created, for example hook into publish_post first, note the ID, and add to this hook from there.