Cover the replication failure scenario and fix the metrics

The failed replication fetch was silently dropping the event
without any possibility to detect the failure. Cover the missing
use-case and generate the task/failed metric whenever the
replication does not succeed.

Also consider a success when the replication event does not
result in any ref-specs, which isn't a failure but simply a NOOP.

Change-Id: I9bc6127d4f9e237772a43d787f8b862319e8d84c