mon: gather perf stats on session adds, removes, counts

I found myself wanting to know this on a running cluster.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: gather perf stats on elections

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1939 from ceph/wip-mon

mon: a few basic perfcounter guages

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>