Ceph: Cache Tier Statistics

Quick tip on how to retrieve cache statistics from the a cache pool.

Simply use the admin socket:

```bash $ sudo ceph daemon osd.{id} perf dump ...

 "tier_promote": 1425,
 "tier_flush": 0,
 "tier_flush_fail": 0,
 "tier_try_flush": 216,
 "tier_try_flush_fail": 21,
 "tier_evict": 1413,
 "tier_whiteout": 201,
 "tier_dirty": 671,
 "tier_clean": 216,
 "tier_delay": 16,

... ```