Enable gprc-go debug log messages with GRPC_GO_LOG_SEVERITY_LEVEL

https://github.com/grpc/grpc-go#how-to-turn-on-logging mentions that the GRPC_GO_LOG_SEVERITY_LEVEL environment variable can be set to enable gRPC debugging, but it was not possible to enable this previously. The log level was always overriden because it was too chatty.

To debug gitlab#297359 (closed), we'll need to be able to see the gprc-go logs for both Gitaly and Praefect.