Grpc channel API breaks when using grpc client instrumentor

Describe your environment
I'm using GrpcInstrumentorClient (https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/grpc/grpc.html#usage-client) version 0.30b1 in a python (3.7) flask application. When closing the channel, using a custom wrapper around channel, calls to the grpc.Channel._channel API fail.

Steps to reproduce

1. Instrument with GrpcInstrumentorClient().instrument()
2. In a grpc middleware, or anywhere with access to the grpc channel object, call a method on channel._channel, like channel._channel.check_connectivity_state()

What is the expected behavior?
the grpc channel._channel is an instance of grpc._cython.cygrpc.Channel

What is the actual behavior?
the grpc channel._channel is an instance of grpc._channel.Channel

Additional context
GRPCInstrumentorClient works fine with no middleware or other interceptors present.

Start with the GrpcInstrumentorClient instrumentation and its channel wrapper, reproducing the failure with Python 3.7, Flask, grpc version 0.30b1, and a call to channel._channel.check_connectivity_state(). Done means instrumented channels retain the expected grpc._cython.cygrpc.Channel API while middleware or other interceptors are present.