Caffeine instrumentation: add weighted size metric

[jhominal]

There are two modifications in this PR:

• caffeine_cache_eviction_weight has been changed from a gauge to a counter - that seems semantically more pertinent to me, as that statistic can only increase (though there is a debate on that point - see Add weighted size counter to caffeine instrumentation #1246);
• caffeine_cache_weighted_size has been added;

There are at least two backwards compatibility concerns that should be given a thought:

1. If the change of type of caffeine_cache_eviction_weight is accepted, users would have to migrate to using the _total suffix;
2. If someone already uses caffeine-instrumentation, and has defined their own caffeine_cache_weighted_size metric, upgrading to a version that includes this PR would break their application;

Point 1 obviously depends on whether the type change is accepted, both from a theoretical point of view (how it should be modelled as a metric), and from a practical point of view (is the change worth a migration for current users).
For point 2, I guess my main point would be to understand the project's policy on backward compatibility when adding metric names in instrumentation libraries. I would be happy to add documentation pertaining to:

• Guava and Caffeine instrumentation, and backwards compatibility policy as it pertains to new metric added by these instrumentation libraries in the future;
• Backwards compatibility policy around adding new metrics to JVM instrumentation;

[zeitlinger]

LGTM