How do I test it? I'm supposed to make ceph unreachable and then I'll get the message, right? Suggestions about making ceph unreachable?

yes make ceph unreacheable and the exception raise should be a bit different.

You can stop all ceph-mon processes.

So, the exact scenario for testing the fix:
on controller-0:
    sudo systemctl stop ceph-mon

on controller-1:
    sudo systemctl stop ceph-mon

on controller-2:
    sudo systemctl stop ceph-mon

Wait 3 minutes, then

on controller-0:
    sudo docker restart gnocchi_metricd
    
cat /var/log/containers/gnocchi/gnocchi-metricd.log
...
2018-12-12 14:04:19,515 [25] ERROR    gnocchi.utils: Unable to initialize storage driver
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/tenacity/__init__.py", line 333, in call
    result = fn(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/gnocchi/storage/__init__.py", line 102, in get_driver
    conf.storage)
  File "/usr/lib/python2.7/site-packages/gnocchi/storage/ceph.py", line 52, in __init__
    self.rados, self.ioctx = ceph.create_rados_connection(conf)
  File "/usr/lib/python2.7/site-packages/gnocchi/common/ceph.py", line 69, in create_rados_connection
    conn.connect()
  File "cradox.pyx", line 843, in cradox.Rados.connect (cradox.c:11327)
TimedOut: [errno 110] error connecting to the cluster

The line "ERROR    gnocchi.utils: Unable to initialize storage driver" is seen and not hidden anymore, so Fixed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2019:0045