When querying subscriptions for a given product id, the billing_account_id should be included in the result. This will enable filtering the subscription list on the GUI side without separate calls every time the billing_account_id is changed. 

https://github.com/RedHatInsights/rhsm-subscriptions/blob/9242a51501aeb860413eb6e9328c66e533cc1642/api/rhsm-subscriptions-api-v2-spec.yaml#L290

Done Criteria: 

• billing_account_id is included in the response & is filled in if this is a metered subscription.
• If there are two metered subscriptions with different billing_account_id's then both are listed as separate entries