An error, if the message was not handed off to FCM successfully.
Optional messageId
messageId:string
A unique message ID string, if the message was handed off to FCM for
delivery.
success
success:boolean
A boolean indicating if the message was successfully handed off to FCM or
not. When true, the messageId attribute is guaranteed to be set. When
false, the error attribute is guaranteed to be set.
Interface representing the status of an individual message that was sent as part of a batch request.