Verified with 4.2.0-0.nightly-2020-03-08-215456 and the issue has been fixed.
$ oc -n openshift-ingress delete secret router-metrics-certs-default
$ oc -n openshift-ingress logs router-default-7f9886d584-crgb6
I0309 06:45:36.222501 1 template.go:637] Reloaded metrics certificate for cert file "/etc/pki/tls/metrics-certs/tls.crt" and key file "/etc/pki/tls/metrics-certs/tls.key"
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.