Add metadata to invoice generated by subscription creation

I am trying to add metadata to all my invoices and so far I was only able to add it to Invoices that I create as not part of a subscription. When I create a subscription, Stripe automatically generates an invoice (first invoice). The creation method of the subscription accepts a metadata object but that is for the subscription object only. The invoice that is generated by the subscription creation is not accessible so that I can add the metadata to it.

How can this be done?