mgr, mgr/telemetry: add access to osd commands in mgr and workload metrics to telemetry

mgr, mgr/telemetry: add access to osd commands in mgr and workload metrics to telemetry

In the manager module (src/pybind/mgr/mgr_module.py), there was a command that allowed access to monitor commands, but there was nothing to access osd commands. I added a new method called osd_command that allows access to the output of commands like "ceph osd.0 perf histogram dump".

In the telemetry module (src/pybind/mgr/telemetry/module.py), I added information that may help to characterize a workload. These changes include:
	- io rate: delta in pg stats since last report, including a pg stats summary and some store stats
	- stats sum per pool: summed up stats from every pg in a pool
	- perf histograms: histogram dump of latency and request size for each osd

Signed-off-by: Laura Flores <lflores@redhat.com>