Ability to add metadata to customers

[goraxan]

Description

We have an integration between Stripe and a third party software which requires the Stripe customers to have some metadata fields.

In the commerce-stripe plugin there's no event that allows us to hook in and modify the customer object to add the required metadata fields.
Would be an option to create a StripeGateway::EVENT_BUILD_CUSTOMER_REQUEST (similar to the existing StripeGateway::EVENT_BUILD_GATEWAY_REQUEST) to allow us to include our metadata fields?

https://github.com/craftcms/commerce-stripe/blob/master/src/services/Customers.php#L59

Thanks