Firestore: add synth metadata.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Running generator for google/firestore/artman_firestore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/firestore-v1beta1.
synthtool > Replaced 'from google.cloud import firestore_v1beta1' in tests/unit/gapic/v1beta1/test_firestore_client_v1beta1.py.
synthtool > Replaced 'client = firestore_v1beta1.FirestoreClient' in tests/unit/gapic/v1beta1/test_firestore_client_v1beta1.py.
synthtool > Cleaned up 0 temporary directories.
synthtool > Wrote metadata to synth.metadata.