prefix_get_custom_meta.php

/**
 * Retrieve custom meta for use in the REST API
 *
 * @param array $object Details of current post.
 * @param string $field_name Name of field.
 * @param WP_REST_Request $request Current request
 *
 * @return mixed
 */
function prefix_get_custom_meta( $object, $field_name, $request ) {
    return get_post_meta( $object[ 'id' ], $field_name, true );
}