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Error calling publish_event with rawPayload metadata

See original GitHub issue

Expected Behavior

Using the Python sample from the documentation to publish messages to a MQTT pub/sub binding with rawPayload=true :

from dapr.clients import DaprClient

with DaprClient() as d:
    req_data = {
        'order-number': '345'
    }
    # Create a typed message with content type and body
    resp = d.publish_event(
        pubsub_name='pubsub',
        topic='TOPIC_A',
        data=json.dumps(req_data),
        metadata=(
                     ('rawPayload', 'true')
                 )
    )
    # Print the request
    print(req_data, flush=True)

Actual Behavior

The following error occurs and the data is not published:

== APP ==     resp = d.publish_event(
== APP ==   File "/home/vscode/.local/lib/python3.8/site-packages/dapr/clients/grpc/client.py", line 350, in publish_event
== APP ==     _, call = self._stub.PublishEvent.with_call(req, metadata=metadata)
== APP ==   File "/home/vscode/.local/lib/python3.8/site-packages/grpc/_channel.py", line 955, in with_call
== APP ==     state, call, = self._blocking(request, timeout, metadata, credentials,
== APP ==   File "/home/vscode/.local/lib/python3.8/site-packages/grpc/_channel.py", line 926, in _blocking
== APP ==     call = self._channel.segregated_call(
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 498, in grpc._cython.cygrpc.Channel.segregated_call
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 366, in grpc._cython.cygrpc._segregated_call
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 360, in grpc._cython.cygrpc._segregated_call
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 218, in grpc._cython.cygrpc._call
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 246, in grpc._cython.cygrpc._call
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 89, in grpc._cython.cygrpc._operate
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/tag.pyx.pxi", line 64, in grpc._cython.cygrpc._BatchOperationTag.prepare
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/operation.pyx.pxi", line 37, in grpc._cython.cygrpc.SendInitialMetadataOperation.c
== APP ==   File "src/python/grpcio/grpc/_cython/_cygrpc/metadata.pyx.pxi", line 41, in grpc._cython.cygrpc._store_c_metadata
== APP == ValueError: too many values to unpack (expected 2)

Checking the source code at client.py the rawPayload metadata probably needs to be assigned to the metadata of the PublishEventRequest, which is not implemented.

 content_type = ""
        if data_content_type:
            content_type = data_content_type
        req = api_v1.PublishEventRequest(
            pubsub_name=pubsub_name,
            topic=topic_name,
            data=req_data,
            data_content_type=content_type)

        # response is google.protobuf.Empty
        _, call = self._stub.PublishEvent.with_call(req, metadata=metadata)

Steps to Reproduce the Problem

Run the sample code from the documentation. In the sample there is another small issue, that the parameter topic needs to be renamed to topic_name.