It seems to me that having a specific UI toolkit font would not help you in your case.
If Go maintainers decide to change the font metrics you will have the same problem. Distributing your own font (as you have mentioned) is the right way to go.
If the Go maintainers change metrics, you can still distribute the old version, or even modify the new version back to the metrics you want and distribute the result. That's the advantage to friendly licensing.
If Go maintainers decide to change the font metrics you will have the same problem. Distributing your own font (as you have mentioned) is the right way to go.