Added a constructor for provided Prom CollectorRegistry.

[ithinkicancode]

Added a constructor for provided Prom CollectorRegistry, plus lib upgrades, minor code enhancement and modernization.

TODOs:

• [ ] Add unit tests for prometheus2 implementation
• [ ] Move example in prometheus2 into mdoc
• [ ] Add documentation for prometheus2 implementation

[ithinkicancode]

Hey @toxicafunk, since you merged my PR already, I'll open a new one to work on the following todo's:

TODOs:

Add unit tests for prometheus2 implementation Move example in prometheus2 into mdoc Add documentation for prometheus2 implementation