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

首先定位 CryptoMaterialsCache 类，并查看其缓存条目和访问路径是如何管理的。将所请求的 size()、snapshot()、hit 和 miss 指标与链接的 Caffeine 和 Guava 示例进行比较。当缓存以有文档说明且可用的形式提供所请求的统计信息，并且相关行为已由测试覆盖时，即视为完成。

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