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