`TypeError: Unexpected type of callable` when REST transport is used with Async clients

[parthea]

See the reproduction code below, and the stack trace that appears when setting the transport argument of DocumentServiceAsyncClient to "rest"

import asyncio
import json

from google.cloud import discoveryengine_v1

doc_json = {
    "title": "Test document title",
    "body": "Test document description",
    "categories": ["sports > clip","sports > highlight"],
    "uri": "http://www.example.com",
}

document = discoveryengine_v1.Document()
document.json_data = json.dumps(doc_json)

client = discoveryengine_v1.DocumentServiceAsyncClient(transport="rest")
request = discoveryengine_v1.CreateDocumentRequest(
    parent="projects/python-docs-samples-tests/locations/global/dataStores/test-structured-data-engine/branches/default_branch",
    document=document,
)
asyncio.run(client.create_document(request=request))

Traceback (most recent call last):
  File "/usr/local/google/home/partheniou/git/google-cloud-python/check.py", line 21, in <module>
    asyncio.run(client.create_document(request=request))
  File "/usr/local/google/home/partheniou/.pyenv/versions/3.9.16/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/local/google/home/partheniou/.pyenv/versions/3.9.16/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "/usr/local/google/home/partheniou/.pyenv/versions/3.9.16/envs/py39/lib/python3.9/site-packages/google/cloud/discoveryengine_v1/services/document_service/async_client.py", line 640, in create_document
    rpc = gapic_v1.method_async.wrap_method(
  File "/usr/local/google/home/partheniou/git/python-api-core/google/api_core/gapic_v1/method_async.py", line 43, in wrap_method
    func = grpc_helpers_async.wrap_errors(func)
  File "/usr/local/google/home/partheniou/git/python-api-core/google/api_core/grpc_helpers_async.py", line 209, in wrap_errors
    raise TypeError("Unexpected type of callable: {}".format(type(callable_)))
TypeError: Unexpected type of callable: <class 'google.cloud.discoveryengine_v1.services.document_service.transports.rest.DocumentServiceRestTransport._CreateDocument'>

If support for REST transport in async clients is not actively being worked on, the error message should say that REST transport is not supported in async clients via a NotImplementedError.