Use this way to extend data to rest api

use Automattic\WooCommerce\StoreApi\Schemas\V1\ProductSchema;

add_action('rest_api_init', function() {
 woocommerce_store_api_register_endpoint_data(
	array(
		'endpoint' => ProductSchema::IDENTIFIER,
		'namespace' => 'plugin_namespace',
		'data_callback' => 'my_data_callback',
		'schema_callback' => 'my_schema_callback',
		'schema_type' => ARRAY_A,
		)
	);
});


function my_data_callback() {
	return [
		'custom-key' => 'custom-value';
	]
}

function my_schema_callback() {
	return [
		'custom-key' => [
			'description' => __( 'My custom data', 'plugin-namespace' ),
			'type' => 'string',
			'readonly' => true,
		]
	];
}