Monitoring: add 'dropped_labels', 'span_context', plus docstring changes

[dpebot]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/monitoring/artman_monitoring.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/monitoring-v3.
synthtool > No replacements made in google/cloud/*/gapic/*_client.py for pattern routing_header\(, maybe replacement is not longer needed?
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/alert_policy_service_client.py.
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/group_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 > No replacements made in google/cloud/*/gapic/*_client.py for pattern (^        )(routing_header = google.api_core.gapic_v1.routing_header.to_grpc_metadata\(
)(\s+)(\[\('[a-z\_]*?\.name', )([a-z\_]*?)(.name\)\], \)
)(\s+metadata.append\(routing_header\)
), maybe replacement is not longer needed?
synthtool > Replaced '(^.*$\\n)*' in google/cloud/monitoring_v3/proto/common_pb2.py.
synthtool > Replaced 'then a new `\\[CONDITION_ID\\]` is created.\n' in google/cloud/monitoring_v3/gapic/alert_policy_service_client.py.
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/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 '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 '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.