Add weighted size counter to caffeine instrumentation

[jhominal]

Hello,

I am using caffeine in a context where I am using a Weigher to bound the cache instance memory consumption.

In that context, I find that the weighted size (which can be found on the EvictionPolicy object in caffeine) is an interesting statistic to track.

I would be happy to author a PR to add that stat to caffeine instrumentation. (guava cache does not seem to expose an API that would make that statistic available)

[zeitlinger]

Great - PR would be welcome 😄