mon: PGMap: keep track of per-pool stats deltas

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: PGMap: reuse existing summary functions to output pool stats

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: PGMap: rework client IO rate calc and output

Create a function so we can use the same format when outputting per-pool
stats

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: OSDMonitor: add 'osd pool stats' command

Fixes: #6147

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #596 from ceph/wip-6147

mon: keep track of per-pool stats

Reviewed-by: Sage Weil <sage@inktank.com>
mon/PGMonitor: set floor below which we do not warn about objects/pg

If a cluster has very few objects, do not generate warnings when the
objects/pg for a pool diverges from the cluster average.  This avoids
spurious errors when you have a relatively empty cluster and a lone pool
with a modest number of objects is too far off the (mostly meaningless)
cluster-wide average.

Fixes: #6521
Signed-off-by: Sage Weil <sage@inktank.com>