Add optional metadata to a charge

[benjamindell]

When making a one off charge (though I suspect this could also be the case for subscription payments too), it is often useful to send a custom field to Stripe so that it is attached to that specific charge (i.e. such as the ID of the product that you have charged the customer for). Your customer.charge method does not allow the metadata argument to be passed. Is this something you'd consider adding? Right now it looks like we'll have to add this in ourselves which i'd ideally prefer not to do.

[paltman]

👍 on supporting metadata access

[blueyed]

Metadata should be supported for all objects.
There is some WIP in #536, where it has been done for subscriptions, but it should really be done in a generic way.