- filters live under filters
- lb_policy, resolver implementations (being part of client_channel) live under client_channel

There is a behavioral discrepency in
`DescriptorPool.FindFileContainingSymbol` between the two
implementations of the protobuf python library (one of which is purely
python, and the other utilizes a python extension library written in
C++).

protobuf on PyPI is distributed via various prebuilt wheels, and our CI
setup fetches the relevant one for each platform.  Our Linux instances
end up fetching a pre-compiled C++ version of the library, whereas our
Mac and Windows builders automatically get the pure python version.
This test fails (and our reflection implementation itself) succeeds with
the C++ implementation but fails with the pure python version.

The test will be temporarily disabled pending a fix for
https://github.com/google/protobuf/issues/2882

While not strictly required by the protocol,
it will make testing easier.

Fix and un-skip the test for _file_containing_extension
in python reflection service to refer to the correct proto
and expect the right object to be returned. Enable it too.

Just the API and a bare-bone implementation

If the server is already serving max requests, return RESOURCE_EXHAUSTED