ho messo questo dento al functions.php del tema corrente attivo:
codice:
add_action('rest_api_init', 'create_api_posts_meta_field');

function create_api_posts_meta_field() {
    // register_rest_field ( 'name-of-post-type', 'name-of-field-to-return', array-of-callbacks-and-schema() )
register_rest_field('post', 'post-meta-fields', array(
            'get_callback' => 'get_post_meta_for_api',
            'schema' => null,
        )
    );
}

function get_post_meta_for_api($object) {
    //get the id of the post object array
$post_id = $object['id'];
    //return the post meta
return get_post_meta($post_id);
}
adesso vedo i records sulle API.