Allow collectors to pass absolute metric paths

They may do it by starting metric paths with
collectors.ABSOLUTE_PATH_MARKER.

This is useful for monitoring things which are
not associated with a particular host, such
as cluster state in a distributed system.
Make Ceph collector to pick up cluster name

Previously the prefix on admin socket names
was a config setting, had to be manually set
for one and only one cluster name.  In the
output stats, the cluster name was not used,
'ceph' was always passed through.

This change makes the collector gather stats
for all clusters it sees, and include the
name of the cluster in the metric names.
Extend Ceph collector to collect pool stats

These stats are only present in more recent versions
of Ceph (currently >=0.72, but expected to be backported).

Because these stats are for cluster-wide pools rather than
an individual host/service, they are reported with a global
name, ceph.<cluster name>.pool.<pool id>.*