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.

ssl-expiration

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.)

Deploy in Openshift

Review ocp-deploy.yaml and then deploy in Openshift

oc apply -f ocp-deploy.yaml

Run container locally

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

Run binary locally

SSL_EXPIRATION_DOMAIN=github.com cargo r

In another terminal

curl localhost:8080/metrics

Build container image

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

Multiple domains

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