How to add meta data to an invoice in Laravel Cashier?

As stated in the issues on Laravel Cashier. It does not state how to add meta data to invoices/charges.

Auth::user()->invoiceFor('Stickers', 500, ['metadata' => ['VAT' => 'TEST']]);

After following how to do this based off the issue, as seen above, it successfully creates the invoice however, the meta data is not added.

How can I add meta data?

via Chebli Mohamed