Make -grpc_prometheus work when not using a grpc server

[henryr]

--grpc_prometheus enables server and client gRPC metrics. But if we don't have a gRPC server, but call servenv.Run() (which we are doing in some internal tools to have the servenv utilities available but without a gRPC server) the process will crash because of some uninitialized server state. Fix by moving the prom enable logic after the check to see if we actually have a server.

[vitess-bot[bot]]

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This PR was closed because it has been stale for 7 days with no activity.