Fix plugins for hostname-free metrics

When porting StackLight to MK, we missed an important code change [1]
that fixes the metrics not associated to a particular host. This change
is backporting the fix in the current code base.

[1] https://review.openstack.org/#/c/378689/

Change-Id: I177db6f68e23ac4fc7feb45df1f30be1fe124acc
3 files changed