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>