praefect: include tracing middleware on client

Currently gitaly spans are not nested under the praefect spans in our tracing. The two spans appear side-by-side.

Here is an example of what that looks like:

That happens because we're not including the grpc tracing middleware on the client connection. Thus the parent span is not correctly propagated by the grpc-proxy.

This patch adds that middleware, giving us the correct nesting, and an additional span for the grpc client within praefect.

Result: