If you have custom secrets you wish to monitor (you created them as k8s resources), the examples in this directory will help you achieve that.
There is a service defined in service.yaml which is used by prometheus service monitor with the config in service-monitor.yaml. The prometheus config assumes you are using the prometheus-operator helm chart
On line 27 of the deployment file you will see the additional options passed to the cert exporter that allows for monitoring certificates based on labels. The example secret is configured to be monitored by prometheus.
NOTE: The label only has to have a matching key. Any value supplied will work!
By default, the certificate dashboard monitors for cert_exporter_cert_expires_in_seconds and cert_exporter_kubeconfig_expires_in_seconds. To have it monitor for secrets, you need to add/change to check cert_exporter_secret_expires_in_seconds.
There could be a number of things wrong, but this helpful flowchart will help to get you sorted.
You can also exec into a pod running in your cluster and try to curl the metrics endpoint of the cert-exporter by running curl cert-exporter:8080/metrics and see what you get back. (cert-exporter hostname assumes that is the name you have used in service.yaml)