Batch mutations size #4423

Hi,
I'm currently using "batchWrite" and i was surprised to see that it's not currently possible to access the mutations_list's size. As it's limited to 500 mutations, we should be able to know the size :)

Environment details

• OS: W10
• Java version: 1.8_191
• google-cloud-java version(s): 0.79.0-beta

Steps to reproduce

N/A

Stacktrace

N/A

Code snippet

var batch: WriteBatch = this.firestore.batch()
// batch.getMutationsSize() ?

External references such as API reference guides used

https://cloud.google.com/firestore/docs/manage-data/transactions#batched-writes

Any additional information below

I'll provide a simple pull request to do it, but i'm pretty sure you don't need me on this ;)
[EDIT] pull request provided

Regards,
Gremi