I need to store a large number of fields, like for a star rating system, but firestore only allows 20,000 fields per document. Is there a known way around this? Right now I am going to 'shard' the fields in multiple documents, and keep the size of each document in a documentSizeTracker document that I use to determine which document to shard to (and add to the counter with a transaction). Is this the correct approach? Any problems with this?