Monitoring: pick up enum fixes in the GAPIC generator.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > You are running the synthesis script directly, this will be disabled in a future release of Synthtool. Please use python3 -m synthtool instead.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:2f6b261ee7fe1aedf238991c93a20b3820de37a343d0cacf3e3e9555c2aaf2ea
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/monitoring/artman_monitoring.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/monitoring-v3.
synthtool > Replaced 'def .*\\(([^\\)]+)\n.*metadata=None\\):\n\\s+"""(.*\n)*?\\s+"""\n' in google/cloud/monitoring_v3/gapic/notification_channel_service_client.py.
synthtool > Replaced 'def .*\\(([^\\)]+)\n.*metadata=None\\):\n\\s+"""(.*\n)*?\\s+"""\n' in google/cloud/monitoring_v3/gapic/metric_service_client.py.
synthtool > Replaced 'def .*\\(([^\\)]+)\n.*metadata=None\\):\n\\s+"""(.*\n)*?\\s+"""\n' in google/cloud/monitoring_v3/gapic/alert_policy_service_client.py.
synthtool > Replaced 'def .*\\(([^\\)]+)\n.*metadata=None\\):\n\\s+"""(.*\n)*?\\s+"""\n' in google/cloud/monitoring_v3/gapic/uptime_check_service_client.py.
synthtool > Replaced 'def .*\\(([^\\)]+)\n.*metadata=None\\):\n\\s+"""(.*\n)*?\\s+"""\n' in google/cloud/monitoring_v3/gapic/group_service_client.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/monitoring_v3/proto/common_pb2.py.
synthtool > No replacements made in google/cloud/monitoring_v3/gapic/alert_policy_service_client.py for pattern then a new `\[CONDITION_ID\]` is created.
, maybe replacement is not longer needed?
synthtool > Replaced '                ::\n\n' in google/cloud/monitoring_v3/gapic/alert_policy_service_client.py.
synthtool > Replaced '^(\\s+)have an ``id`` label:  ::      resource.type =\n.*' in google/cloud/monitoring_v3/proto/metric_service_pb2.py.
synthtool > Replaced 'return google.api_core.grpc_helpers.create_channel\\(\n(\\s+)address,\n\\s+credentials=.*,\n\\s+scopes=.*,\n' in google/cloud/monitoring_v3/gapic/transports/notification_channel_service_grpc_transport.py.
synthtool > Replaced 'return google.api_core.grpc_helpers.create_channel\\(\n(\\s+)address,\n\\s+credentials=.*,\n\\s+scopes=.*,\n' in google/cloud/monitoring_v3/gapic/transports/alert_policy_service_grpc_transport.py.
synthtool > Replaced 'return google.api_core.grpc_helpers.create_channel\\(\n(\\s+)address,\n\\s+credentials=.*,\n\\s+scopes=.*,\n' in google/cloud/monitoring_v3/gapic/transports/uptime_check_service_grpc_transport.py.
synthtool > Replaced 'return google.api_core.grpc_helpers.create_channel\\(\n(\\s+)address,\n\\s+credentials=.*,\n\\s+scopes=.*,\n' in google/cloud/monitoring_v3/gapic/transports/group_service_grpc_transport.py.
synthtool > Replaced 'return google.api_core.grpc_helpers.create_channel\\(\n(\\s+)address,\n\\s+credentials=.*,\n\\s+scopes=.*,\n' in google/cloud/monitoring_v3/gapic/transports/metric_service_grpc_transport.py.
synthtool > Replaced 'from google.cloud.monitoring_v3.gapic import notification_channel_service_client\n' in google/cloud/monitoring_v3/__init__.py.
synthtool > Replaced 'notification_channel_service_client.NotificationChannelServiceClient' in google/cloud/monitoring_v3/__init__.py.
synthtool > Cleaned up 1 temporary directories.