With FPSmeter you can measure the performance of your personal game server. It uses different values like the FPS, Packet loss and the latency to determine the performance. You can grab the result in an HTML format or via JSON with an API.
With the launch of Just Cause 3 Multiplayer FPSmeter.org is directly supported. Just give it a try.
Please provide all necessary informations about your game server. Be sure that you are providing the correct Query port and not the connect port, these ports can be different.