Register grpc metrics in main

This needs to be registered last after all other services have been
registred.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/containerd/main.go

@@ -147,6 +147,8 @@ func main() {
 				return err
 			}
 		}
+		// register metrics last after all other services
+		grpc_prometheus.Register(server)
 		log.G(global).Info("starting GRPC API server...")
 		if err := serveGRPC(server); err != nil {
 			return err