Allow access to grpc channel api with the client interceptor

[ianmcnally]

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This would close #1697. The interceptor channel as is breaks access to channel private attributes, like Channel._channel. To allow it, I wrapped the incoming channel with a getattr that tries Channel._channel if it isn't present in Channel.

Description

It's described in #1697 , but the gist is:

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.

Fixes #1697

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Instrument with GrpcInstrumentorClient().instrument() 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()

• [x] Test A

Does This PR Require a Core Repo Change?

• [ ] Yes. - Link to PR:
• [x] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added