its a very, very simple update alert to add to Prometheus to monitor that, and alert if the cert is within so many days of expiring. You need the 'blackbox exporter' and a simple rule such as:

  alert: TlsCertExpiringSoon
  expr: (probe_ssl_earliest_cert_expiry
    - time()) < (86400 * 14)
  for: 10m
    labels:
    product: Name_of_Product
    severity: page
  annotations:
    description: the tls cert for the URL {{ $labels.instance }} expires in less than 14 days!
  summary: TLS cert for {{$labels.instance}} expiring