Emit stats for cache hits in addition to misses, retrievals, etc in ExpirationKCVSCache

We currently only emit stats for retrievals, misses, expirations, however we do not emit stats for hits explicitly. Without hits, displaying the cache hit ratio is very awkward in systems like graphite. Also, it's easier to reason about hit ratio than miss ratio.

Start by locating ExpirationKCVSCache and tracing where retrieval, miss, and expiration statistics are emitted. Add an explicit hit statistic alongside those existing events so cache hit ratios can be displayed directly. Done means cache hits are reported consistently with the other cache statistics.

Written by the indexing model from the issue text.