[C++][Flight] Show example of using Channelz for debugging

Show how to setup channelz support in a Flight server and how to query using https://github.com/grpc-ecosystem/grpcdebug

Start by locating the existing C++ Flight server examples and reviewing how Channelz support is exposed. Use grpcdebug to query the running server, and consider the example complete when it shows both Channelz setup in a Flight server and a working debugging query.

Written by the indexing model from the issue text.