CacheMetrics: expose request count for cache sizing decisions

Cache hit ratio alone is insufficient for deciding which caches should
be grown. A rarely accessed cache may have a poor hit ratio because most
lookups are first-time accesses, so increasing its size provides little
benefit.

Expose the total request count alongside existing cache metrics so cache
growth decisions can be prioritized based on both hit ratio and access
volume. This is especially useful when storage constraints prevent
growing every cache, allowing heavily used caches with lower hit ratios
to be prioritized over infrequently accessed caches.

Release-Notes: Emit caches/memory_request_count and caches/disk_request_count metrics
Change-Id: I4714615892e4b8c89a040851760539d3fab6bb28