Transcoding fails when HTTP request body contains a field name which is a reserved word in python

[parthea]

There is an error in googleapis/python-debugger-client#151 which occurs in the transcoding logic when the name of the request field whose value is mapped to the HTTP request body is a reserved word in python. In the case of python-debugger-client, the breakpoint field is used in the HTTP request body here.

See the transcode function below.

python-api-core/google/api_core/path_template.py

Lines 250 to 252 in 2a6a36b

	def transcode(http_options, message=None, **request_kwargs):
	"""Transcodes a grpc request pattern into a proper HTTP request following the rules outlined here,
	https://github.com/googleapis/googleapis/blob/master/google/api/http.proto#L44-L312

Gapic generator python adds a trailing underscore to field names if the name is also a reserved word in Python. A similar issue was reported in #227 and fixed in #228. Based on comment #227 (comment) it is not possible for fields to have a trailing _ at the proto level (Creating a field like this generates an error from the protobuf runtime).

See build log https://github.com/googleapis/python-debugger-client/actions/runs/4292539515/jobs/7479094983 which shows the failure when a request has body with breakpoint_ instead of breakpoint.

>           response = client.set_breakpoint(request)

tests/unit/gapic/debugger_v2/test_debugger2.py:1920: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
google/cloud/debugger_v2/services/debugger2/client.py:547: in set_breakpoint
    response = rpc(
.nox/unit-3-8/lib/python3.8/site-packages/google/api_core/gapic_v1/method.py:113: in __call__
    return wrapped_func(*args, **kwargs)
.nox/unit-3-8/lib/python3.8/site-packages/google/api_core/timeout.py:120: in func_with_timeout
    return func(*args, **kwargs)
.nox/unit-3-8/lib/python3.8/site-packages/google/api_core/grpc_helpers.py:72: in error_remapped_callable
    return callable_(*args, **kwargs)
google/cloud/debugger_v2/services/debugger2/transports/rest.py:722: in __call__
    transcoded_request = path_template.transcode(http_options, pb_request)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

http_options = [{'body': 'breakpoint', 'method': 'post', 'uri': '/v2/debugger/debuggees/{debuggee_id}/breakpoints/set'}]
message = debuggee_id: "sample1"
breakpoint_ {
  id: "id_value"
  action: LOG
  location {
    path: "path_value"
    line: 424
...alue"
      type_: "type__value"
      var_table_index {
        value: 541
      }
      status {
      }
    }
  }
}

[parthea]

Bumping to P2 as this is blocking REST Transport support for 1 client library in googleapis/python-debugger-client#151. The REST Transport support feature has not yet launched for the python-debugger-client client library yet.