fix: add optional generics to metric groups

[achingbrain]

To give a bit more type saftey to metric groups, add optional generics that allow you to specify which grouped metrics you can update/increment.

Change checklist

• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation if necessary (this includes comments as well)
• [ ] I have added tests that prove my fix is effective or that my feature works