[knot-resolver 6] Add new metric

[Jean-Daniel]

A simple feature request. It should be straightforward to add a metrics to expose the actual number of entries in the cache (as returned by cache.count()).

I find it interesting to monitor how fast the cache is filling, and helping sizing it.

[vcunat]

Sounds good to me, except that for this I'd rather expect cache.stats().usage_percent.

[Jean-Daniel]

Right. I guess all stats in cache.stats() may be exposed ;-)