Fix import metrics in notifications/messaging.py (#1254)
Fix import metrics in notifications/messaging.py (#1254)

`from lemur import metrics` is incorrect for notifications/messaging.py
because that is importing the `metrics` module rather than the
instanciated `lemur.extensions.metrics` object.  This will cause errors
if you import notifications/messaging.py elsewhere, since it can cause
circular dependencies.

Change-Id: Ice28c480373601420fc83bae2d27bb6467cdb752
Netflix/lemur,Netflix/lemur,Netflix/lemur,Netflix/lemur