Error if `port` and `metrics-port` are the same

[isker]

I had the misfortune of configuring --port to 8081; later, --metrics-port was added with that default value. Upon picking up this new feature, there were confusing errors on startup about 8081 already being bound. I eventually figured it out 🌞.

Ideally there would be a clear error message when these options are configured with the same value.

[isker]

Thank you!