As of version 1.3.7 the cert-util-operator supports alerting on expiring certs stored as secrets in the cluster so you should probably use that instead. It can also populate Routes with certs stored in secrets.

Checks SSL certificate expiration, exports as Prometheus metric and alerts when expiry is near. (I wrote this for Openshift but it should work in vanilla Kubernetes. I haven't tested it.)

Review ocp-deploy.yaml and then deploy in Openshift

oc apply -f ocp-deploy.yaml

docker pull alpha60/ssl-exporter:latest
docker run --rm -e SSL_EXPIRATION_DOMAIN=github.com -p 80:8080 ssl-exporter

In another terminal

curl localhost/metrics

SSL_EXPIRATION_DOMAIN=github.com cargo r

In another terminal

curl localhost:8080/metrics

cargo b --release
BUILD_KIT=1 docker build -t ssl-exporter .

Just add them as a comma separated list like this: SSL_EXPIRATION_DOMAIN=github.com,microsoft.com,google.com