CA-116417: Added Host_metrics to GC

Issue: The host metrics objects do not get GC'ed currently because of
which they keep accumulating over a period of time for invalid hosts

Fix: Creating two sets of host_metrics objects. One has all the
host_metrics objects both valid and invalid, and second has all
the host_metrics objects only for valid hosts. Removing all the
invalid host_metrics objects from the first set with the help
of second set.
Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>