feature: mapk for Meter

[DocCringol]

I've seen the implementation of mapK for Tracer and thought of doing the same for Meter. I don't pretend that what I'm suggesting is a good implementation though. Also I didn't really get where is the best place to implement tests. So I did only an example one, for counter

Also, there is a point could be made that it is possible to make less amount of boilerplate code (in observable components at least) by encapsulating Builder trait

So, it's more of a feature suggestion

But if somebody will like the idea and will be able to tell me more about project structure then I'm ready to finish this pr

[iRevive]

Thanks for the PR! I will review the changes this week.

[iRevive]

Looks good overall, thanks!

@NthPortal would you like to take a look?