FR Limit creation of collections

### [REQUIRED] Describe your environment

* Operating System version: Windows 10
* Browser version: Chrome 70
* Firebase SDK version: 6.8.0
* Firebase Product: Cloud Firestore

### [REQUIRED] Describe the problem

#### Steps to reproduce:

Sometimes we need limit the creations of documents in a collections, because users can make millons of multiple tasks in the same collections at the same time. We need limit the creation of collections an throws an error if the creation of docs it was surpass, nowadays we can handle with this creating a cloud function but it becomes a high cost bill if we handle with millons of documents creations.

related question:

[How to limit documents in a collection in Firebase Firestore - stackoverflow](https://stackoverflow.com/questions/54282581/how-to-limit-documents-in-a-collection-in-firebase-firestore
)

The request targets Cloud Firestore document-creation limits in the Firebase JavaScript SDK. No source file, test, entry point, or acceptance criterion is identified; first clarify the intended enforcement layer and expected error behavior before locating implementation and tests.

Written by the indexing model from the issue text.