droppedTooManyPendingMessages property

The percentage of accepted messages that were dropped due to [too many undelivered non-collapsible messages](https://firebase.google.com/docs/cloud-messaging/concept-options#collapsible_and_non-collapsible_messages).

Specifically, each app instance can only have 100 pending messages stored on our servers for a device which is disconnected. When that device reconnects, those messages are delivered. When there are more than the maximum pending messages, we call [OnDeletedMessages()](https://firebase.google.com/docs/cloud-messaging/android/receive#override-ondeletedmessages) in our SDK instead of delivering the messages.