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.