Excerpt from init.php for Gutenberg sidebar metadata demo

init.php

<?php
add_action( 'init', function(){
	register_meta( 'post', '_demo_select', array(
 		'type'		=> 'string',
		'single'	=> true,
 		'show_in_rest'	=> true,
		'auth_callback' => function() {
			return current_user_can('edit_posts');
		}
 	) );
	 register_meta( 'post', '_demo_text_field', array(
		'type'		=> 'string',
		'single'	=> true,
		'show_in_rest'	=> true,
		'auth_callback' => function() {
			return current_user_can('edit_posts');
		}
	) );
});