* @property string $id Unique identifier for the object.
* @property string $object String representing the object's type. Objects of the same type share the same value.
* @property null|string $invoice The invoice in which this usage period has been billed for.
* @property bool $livemode Has the value <code>true</code> if the object exists in live mode or the value <code>false</code> if the object exists in test mode.
* @property \Stripe\StripeObject $period
* @property string $subscription_item The ID of the subscription item this summary is describing.
* @property int $total_usage The total usage within this usage period.