Reflections don't work (I may know why)

I use server reflection as defined in [the documentation](https://grpclib.readthedocs.io/en/latest/reflection.html) and I have a single service called `helloworld.Greeter`.

gRPC call `list_services: "*"` is processed fine and returns a single service `helloworld.Greeter`.

However, `file_containing_symbol: "helloworld.Greeter"` only returns an error `not found`

Throughout my own investigation I found that error can be traced to
[grpclib/reflection/_deprecated.py:L79](https://github.com/vmagamedov/grpclib/blob/master/grpclib/reflection/_deprecated.py#L79)
Where `self._pool` does not have any descriptors for `helloworld.Greeter` or any of its methods

I suspect that [THIS](https://github.com/vmagamedov/grpclib/blob/master/examples/helloworld/helloworld_pb2.py#L17) is how the descriptors should be added in order for reflections to work with betterproto

まず、issue に記載されている reflection 呼び出しを再現し、次にリンクされた lookup の grpclib/reflection/_deprecated.py と、リンクされた descriptor セットアップの examples/helloworld/helloworld_pb2.py を調査します。file_containing_symbol が helloworld.Greeter に対して正常に解決され、list_services が引き続きサービスを返せば完了です。

索引モデルが issue の本文から書いたものです。