pkg/metrics: introduce v2

[lukedirtwalker]

Introduce a v2 version of pkg/metrics. The current metrics package is not optimal:

• Using objects that have been called with With allocated on each increment/set value call.
• Having the option to call With ties consumer code with initialization code.
• There is no way whether a counter is fully initialized with all labels.

A full discussion can be found in https://github.com/scionproto/scion/pull/4527.

The new v2 version is compatible with prometheus, so no explicit conversion functions are offered, instead the prometheus creation functions can be used directly.

So far the new library has been plugged in selected places. The intention is that it will eventually replace the current library completely, however doing so in a single sweep would be too extensive.

[matzf]

This change is