Ensure that firestore-counter only writes to max 100 documents inside a single transaction

[laurenzlong]

[READ] Step 1: Are you in the right place?

Issues filed here should be about a feature request for a specific extension in this repository. To file a feature request that affects multiple extensions or the Firebase Extensions platform, please reach out to
Firebase support directly.

[REQUIRED] Step 2: Extension name: firestore-counter

Currently, in the worst case, firestore-counter will act on 499 documents at once since that's the shard limit. (https://github.com/firebase/extensions/blob/master/firestore-counter/functions/src/worker.ts#L32). This is not recommended by the Firestore team. A transaction will have decreased performance if it writes/deletes more than 100 documents inside a transaction.

The code needs to be modified to ensure no more than 100 documents are touched inside a single transaction. It might require changing the number of plans: https://github.com/firebase/extensions/blob/master/firestore-counter/functions/src/worker.ts#L186

[lepatryk]

The easiest way to achieve it is to lower the max number of shards to 100.

Here's a PR to do that: #149

[laurenzlong]

Thanks!