Measure cache hits and misses ratios

[isaranchuk]

Add implementation of CacheStatisticsProvider to instrument cache hits and misses.
It will require changes to MemcachedCache to count cache hits and misses, so this information can be exposed via /metrics endpoint.

[igorbolic]

This would be a nice feature to have, and we are planing to add the full cache metrics in the foreseeable future.
Rudimentary, cache metrics for hits & misses (per Spring Boot instance), could be easily added to the MemcachedCache, and probably shipped with the next release.