Container: Add routing header to method metadata, add nox session docs (via synth).

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/container/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:d3df563538225ac6caac45d8ad86499500211d1bcb2536955a6dbda15e1b368e
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/container/artman_container_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/container-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/container/v1/cluster_service.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/container-v1/google/cloud/container_v1/proto/cluster_service.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/container-v1/google/cloud/container_v1/proto.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/container_v1/proto/cluster_service_pb2.py.
synthtool > Replaced 'nodePool>\\n' in google/cloud/container_v1/proto/cluster_service_pb2.py.
synthtool > Replaced '(\\s+)`__ instead' in google/cloud/container_v1/proto/cluster_service_pb2.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/container/.nox/blacken
pip install black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/gapic/transports/cluster_manager_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/gapic/cluster_manager_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/proto/cluster_service_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/gapic/cluster_manager_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/tests/unit/gapic/v1/test_cluster_manager_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/container/google/cloud/container_v1/proto/cluster_service_pb2.py
All done! ✨ 🍰 ✨
9 files reformatted, 8 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.