Error Reporting: 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/devtools/clouderrorreporting/artman_errorreporting.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/error-reporting-v1beta1.
synthtool > Replaced 'from google.devtools.clouderrorreporting_v1beta1.proto import ' in google/cloud/errorreporting_v1beta1/proto/report_errors_service_pb2_grpc.py.
synthtool > Replaced 'from google.devtools.clouderrorreporting_v1beta1.proto import ' in google/cloud/errorreporting_v1beta1/proto/error_group_service_pb2_grpc.py.
synthtool > Replaced 'from google.devtools.clouderrorreporting_v1beta1.proto import ' in google/cloud/errorreporting_v1beta1/proto/error_group_service_pb2.py.
synthtool > Replaced 'from google.devtools.clouderrorreporting_v1beta1.proto import ' in google/cloud/errorreporting_v1beta1/proto/error_stats_service_pb2.py.
synthtool > Replaced 'from google.devtools.clouderrorreporting_v1beta1.proto import ' in google/cloud/errorreporting_v1beta1/proto/report_errors_service_pb2.py.
synthtool > Replaced 'from google.devtools.clouderrorreporting_v1beta1.proto import ' in google/cloud/errorreporting_v1beta1/proto/error_stats_service_pb2_grpc.py.
synthtool > Replaced '\n        This endpoint accepts either an OAuth token, or an API key for\n        authentication. To use an API key, append it to the URL as the value of\n        a ``key`` parameter. For example:\n\n        \\.\\. raw:: html\n        <pre>POST .*\n' in google/cloud/errorreporting_v1beta1/gapic/report_errors_service_client.py.
synthtool > Replaced '\n        This endpoint accepts either an OAuth token, or an API key for\n        authentication. To use an API key, append it to the URL as the value of\n        a ``key`` parameter. For example:\n\n        \\.\\. raw:: html\n        <pre>POST .*\n' in google/cloud/errorreporting_v1beta1/gapic/transports/report_errors_service_grpc_transport.py.
synthtool > Cleaned up 0 temporary directories.
synthtool > Wrote metadata to synth.metadata.