Dataproc: 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/cloud/dataproc/artman_dataproc_v1beta2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/dataproc-v1beta2.
synthtool > Replaced 'metadata_type=operations_pb2.ClusterOperationMetadata,' in google/cloud/dataproc_v1beta2/gapic/cluster_controller_client.py.
synthtool > No replacements made in google/cloud/dataproc_v1beta2/gapic/cluster_controller_client.py for pattern \s+<strong>Note:</strong>.*
(.*
)+?.*types.FieldMask., maybe replacement is not longer needed?
synthtool > Running generator for google/cloud/dataproc/artman_dataproc_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/dataproc-v1.
synthtool > Replaced 'metadata_type=operations_pb2.ClusterOperationMetadata,' in google/cloud/dataproc_v1/gapic/cluster_controller_client.py.
synthtool > No replacements made in google/cloud/dataproc_v1/gapic/cluster_controller_client.py for pattern \s+<strong>Note:</strong>.*
(.*
)+?.*types.FieldMask., maybe replacement is not longer needed?
synthtool > Replaced '# Generated by the protocol buffer compiler.  DO NOT EDIT!' in google/cloud/dataproc_v1beta2/proto/clusters_pb2.py.
synthtool > Replaced ', and must\n\\s+conform to the following PCRE regular expression:(.*\n)+?.*No more than 32' in google/cloud/dataproc_v1beta2/proto/workflow_templates_pb2.py.
synthtool > Replaced ', and must conform to\n\\s+the following regular expression:(.*\n)+?.* No more than' in google/cloud/dataproc_v1beta2/proto/workflow_templates_pb2.py.
synthtool > No replacements made in google/cloud/dataproc_v1/gapic/cluster_controller_client.py for pattern }
(\s+Similarly, to change the number of preemptible workers in a cluster to 5,), maybe replacement is not longer needed?
synthtool > Cleaned up 0 temporary directories.
synthtool > Wrote metadata to synth.metadata.