feat(metrics:cache): Add metric for estimated cache size

[jeqo]

Cache size is not included on default Caffeine stats. Piggybacking on counter to record updates to estimated size.

Reference: https://github.com/micrometer-metrics/micrometer/blob/main/micrometer-core/src/main/java/io/micrometer/core/instrument/binder/cache/CaffeineStatsCounter.java

[AnatolyPopov]

@jeqo could you rebase this one?

[jeqo]

@AnatolyPopov, sure. PR rebased and updated to latest metrics. Take a look, thanks!