gRPC client instrumentation example doesn't run

[alertedsnake]

The example in the gRPC instrumentation for the client side doesn't actually run as is.

Steps to reproduce
I copied the code into a Python file, made up some generated protobuf code for testing, and ran the program.

What is the expected behavior?

Greeter client received: YOU

What is the actual behavior?

  File "/tmp/client.py", line 42, in <module>
    run()
  File "/tmp/client.py", line 32, in run
    with grpc.insecure_channel("localhost:50051") as channel:
  File "/home/michael/work/opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-grpc/src/opentelemetry/instrumentation/grpc/grpcext/_interceptor.py", line 235, in __enter__
    raise NotImplementedError()
NotImplementedError

I'll probably work on this one too, since I need a test client to verify some of the other issues I'm already working on.