fix: Update pubsub_context to correctly modify proto map fields.

[dpcollins-google]

fix: Update pubsub_context to correctly modify proto map fields.

[anguillanneuf]

This still has some issues for publisher_client = make_publisher(topic_path):

Traceback (most recent call last):
  File "publisher_example.py", line 66, in <module>
    publish(
  File "publisher_example.py", line 41, in publish
    publisher_client = make_publisher(topic_path_object)
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/cloudpubsub/make_publisher.py", line 73, in make_publisher
    make_async_publisher(
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/cloudpubsub/make_publisher.py", line 43, in make_async_publisher
    underlying = make_wire_publisher(
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/internal/wire/make_publisher.py", line 53, in make_publisher
    region=topic.location.region,
AttributeError: 'str' object has no attribute 'location'

[dpcollins-google]

This still has some issues for publisher_client = make_publisher(topic_path_object)

This is a separate issue, I'll fix it in a separate PR.

[anguillanneuf]

Sg. The above error was caused by a TopicPath str, when a TopicPath object is used, this is the error:

Traceback (most recent call last):
  File "/Users/tianzi/.virtualenvs/py38/lib/python3.8/site-packages/google/api_core/grpc_helpers.py", line 57, in error_remapped_callable
    return callable_(*args, **kwargs)
  File "/Users/tianzi/.virtualenvs/py38/lib/python3.8/site-packages/grpc/_channel.py", line 826, in __call__
    return _end_unary_response_blocking(state, call, False, None)
  File "/Users/tianzi/.virtualenvs/py38/lib/python3.8/site-packages/grpc/_channel.py", line 729, in _end_unary_response_blocking
    raise _InactiveRpcError(state)
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
        status = StatusCode.INTERNAL
        details = "Received http2 header with status: 400"
        debug_error_string = "{"created":"@1602091143.963493000","description":"Received http2 :status header with non-200 OK status","file":"src/core/ext/filters/http/client/http_client_filter.cc","file_line":130,"grpc_message":"Received http2 header with status: 400","grpc_status":13,"value":"400"}"
>

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "publisher_example.py", line 66, in <module>
    publish(
  File "publisher_example.py", line 41, in publish
    publisher_client = make_publisher(topic_path_object)
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/cloudpubsub/make_publisher.py", line 73, in make_publisher
    make_async_publisher(
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/cloudpubsub/make_publisher.py", line 43, in make_async_publisher
    underlying = make_wire_publisher(
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/internal/wire/make_publisher.py", line 67, in make_publisher
    partition_count = admin_client.get_topic_partition_count(topic)
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite/internal/wire/admin_client_impl.py", line 37, in get_topic_partition_count
    partitions: TopicPartitions = self._underlying.get_topic_partitions(
  File "/Users/tianzi/Projects/python-pubsublite/google/cloud/pubsublite_v1/services/admin_service/client.py", line 475, in get_topic_partitions
    response = rpc(request, retry=retry, timeout=timeout, metadata=metadata,)
  File "/Users/tianzi/.virtualenvs/py38/lib/python3.8/site-packages/google/api_core/gapic_v1/method.py", line 145, in __call__
    return wrapped_func(*args, **kwargs)
  File "/Users/tianzi/.virtualenvs/py38/lib/python3.8/site-packages/google/api_core/grpc_helpers.py", line 59, in error_remapped_callable
    six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "<string>", line 3, in raise_from
google.api_core.exceptions.InternalServerError: 500 Received http2 header with status: 400

[anguillanneuf]

This PR doesn't fix make_publisher(), but approving in preparation for more fixes coming soon.