Make CryptoMaterialsCache return stats

I'd like to collect some metrics on caching for performance optimization and CryptoMaterialsCache currently doesn't provide a way to collect stats such as size, hit, and miss. Could the CryptoMaterialsCache class be wired up to provide something similar to Caffeine or Guava cache such as size() and snapshot()?

Here are some examples:
https://github.com/ben-manes/caffeine/blob/master/caffeine/src/main/java/com/github/benmanes/caffeine/cache/stats/CacheStats.java#L60
https://github.com/google/guava/blob/master/guava/src/com/google/common/cache/LocalCache.java#L4913
https://github.com/google/guava/blob/master/guava/src/com/google/common/cache/AbstractCache.java#L239

Start by locating the CryptoMaterialsCache class and reviewing how its cache entries and access paths are managed. Compare the requested size(), snapshot(), hit, and miss metrics with the linked Caffeine and Guava examples. Done means the cache exposes the requested statistics in a documented, usable form and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.