mds: record metrics from all MDSs in MDS rank 0

mds: record metrics from all MDSs in MDS rank 0

`MetricAggregator` class aggregates metrics from all active ranks
and places metrics appropriately as defined by user queries.

This is implemented as a separate dispatcher since metric update
messages from active MDSs are frequent so as to avoid messages
getting stuck in MDSRank queue (suggested by Patrick).

Signed-off-by: Venky Shankar <vshankar@redhat.com>