I basically add a /global-health endpoint on my server. It executes a bunch of checks programmatically - e.g. database connection, rendering working, etc. It would be easy to add in "fail if cert expires in next month".

Then, I monitor just that one endpoint with Stackdriver (because it's easy). If any of the checks fail, it logs it, prints details, and sets a 500 header code. Adding new checks is just a code change.