Extend metrics library

[lambdanis]

This is a general rewrite of pkg/metrics library, but it a fully backwards compatible way. Goals and key components are described in doc.go, so I would suggest starting the review there :)

For usage examples see #2604 (defining health metrics group, migrating some custom metrics & collectors to the new interface).

Fixes: #2376

[ghost]

I pushed some commits with small fixes. I will squash the commits after the review, just didn't want to force push now.

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[ghost]

If it's not too much hassle, please fold the last commit (metrics: Simplify Group interface) with the second commit (Extend metrics library).

Sure, I think I'll squash all commits following "Extend metrics library" into it. I separated some code reshuffling into the first commit, so that it's clearer what code is new and what is just moved.

[ghost]

I added two commits with fixes, will squash them too.