Add cache keys index metrics

The following metrics were added foe each cache:
* cache/chroniclemap/keys_index_size_<cache-name> for showing the
  current size of the index (that one is one to one with disk entries)
* cache/chroniclemap/keys_index_add_latency_<cache-name> to indicate how
  much time it takes to add key to the index over time
* cache/chroniclemap/keys_index_remove_and_consume_older_than_latency_<cache-name>
  to indicate how much time it takes to remove and consume all keys
  older than expiration time
* cache/chroniclemap/keys_index_remove_lru_key_latency_<cache-name> to
  indicate how much time it takes to remove and consumer LRU key

Bug: Issue 15121
Change-Id: I8141c0f00dfe6e7dc3b06517c3383d19fe883492