The openshift-ingress namespace has no labels, so there's no way to select for it with namespaces.
Nor can you grant access to prometheus.
Verified this bug 4.1.0-0.nightly-2019-05-07-233329
oc get namespaces openshift-ingress --show-labels
NAME STATUS AGE LABELS
openshift-ingress Active 3h24m name=openshift-ingress,network.openshift.io/policy-group=ingress,openshift.io/cluster-monitoring=true
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.