Translate: Add routing header to method metadata, fix docstring (via synth).

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/translate/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:f3a3f88000dc1cd1b4826104c5574aa5c534f6793fbf66e888d11c0d7ef5762e
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis-private.
synthtool > Running generator for google/cloud/translate/artman_translate_v3beta1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis-private/artman-genfiles/python/translate-v3beta1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis-private/google/cloud/translate/v3beta1/translation_service.proto to /home/kbuilder/.cache/synthtool/googleapis-private/artman-genfiles/python/translate-v3beta1/google/cloud/translate_v3beta1/proto/translation_service.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis-private/artman-genfiles/python/translate-v3beta1/google/cloud/translate_v3beta1/proto.
synthtool > Replaced 'google.cloud.translation_v3beta1.proto' in google/cloud/translate_v3beta1/proto/translation_service_pb2_grpc.py.
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/translate/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/gapic/translation_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/gapic/transports/translation_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/proto/translation_service_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/tests/unit/gapic/v3beta1/test_translation_service_client_v3beta1.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/gapic/translation_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/translate/google/cloud/translate_v3beta1/proto/translation_service_pb2.py
All done! ✨ 🍰 ✨
8 files reformatted, 15 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 3 temporary directories.
synthtool > Wrote metadata to synth.metadata.