fix(next): missing metadata on upgrade

Because:

- On a subscription upgrade, `plan_change_date` and `previous_plan_id`
  need to be added to the subscription metadata.

This commit:

- Adds missing `plan_change_date` and `previous_plan_id` to subscription
  updates.

Closes #FXA-11540

Author: StaberindeZA

libs/payments/cart/src/lib/checkout.service.ts

@@ -522,6 +522,10 @@ export class CheckoutService {
         // Note: These fields are due to missing Fivetran support on Stripe multi-currency plans
         [STRIPE_SUBSCRIPTION_METADATA.Amount]: cart.amount,
         [STRIPE_SUBSCRIPTION_METADATA.Currency]: cart.currency,
+        [STRIPE_SUBSCRIPTION_METADATA.PreviousPlanId]: fromPriceId,
+        [STRIPE_SUBSCRIPTION_METADATA.PlanChangeDate]: Math.floor(
+          Date.now() / 1000
+        ),
       },
     });
   }

libs/payments/customer/src/lib/types.ts

@@ -48,6 +48,8 @@ export enum STRIPE_SUBSCRIPTION_METADATA {
   Currency = 'currency',
   Amount = 'amount',
   SubscriptionPromotionCode = 'appliedPromotionCode',
+  PreviousPlanId = 'previous_plan_id',
+  PlanChangeDate = 'plan_change_date',
 }
 
 export enum STRIPE_INVOICE_METADATA {