By having some small portion of calls skip cache and go straight to
disk, we can ensure the cache is always kept fresh and never expires (at
least, for active containers). Previously, when shard ranges fell out of
cache there would frequently be a thundering herd that could overwhelm
the container server, leading to 503s served to clients or an increase
in async pendings.
Include metrics for hit/miss/skip rates.
Change-Id: I6d74719fb41665f787375a08184c1969c86ce2cf
Related-Bug: #1883324
8c6ccb5fd4