mon: test dirty stats in ceph df detail

Signed-off-by: Loic Dachary <loic@dachary.org>
histogram: fix histogram::get_position_micro overflow

Convert the return values to uint64_t

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1146 from dachary/wip-agent

contributions to the "osd: basic cache tier agent" series