Extract client name from Go GRPC client

This is actually from discussion in gitlab#35520 (closed), which traced back to #2053 (closed) where we don't pass client name from golang client. In labkit!50 (merged), after some discussion with @andrewn , we decided to implement an interface similar as correlation ID, where we can pass client name from the client side, This MR is to add the logic to extract it on Gitaly side.

Closes #2053 (closed)