WooCommerce - Add custom post_meta data to the order REST API response.

functions.php

<?php
	
	/**
	 * Hook into order API response to add custom field data.
	 *
	 * @param $order_data   array
	 * @param $order   		WC_Order
	 * @param $fields   	array
	 * @param $server   	array
	 *
	 * @return $order_data	array
	 */
	add_filter( 'woocommerce_api_order_response', 'add_foo_field_order_api_response', 20, 4 );
	function add_foo_field_order_api_response( $order_data, $order, $fields, $server ) {
		
		// Get the custom order post_meta data and add it to the order_data array.
		$order_data['foo'] = get_post_meta( $order->id, 'foo', true );
        
		return $order_data;
	}