An agent-based check for Check_MK, which monitors the remaining days for your Let's Encrypt certificates.
It queries the output from certbot certificates and creates a check for each domain which has a certificate from Let's Encrypt.
Deploy the directory local to your Check_MK instance and place them in your sites directory underneath /opt/omd/sites.
Afterwards deploy the plugin to your hosts where you want to monitor Let's Encrypt certificates. Usually you'll have to put the plugin in /usr/lib/check_mk_agent/plugins.
Now Check_MK should find the check(s) automatically.
Remaining days which will create a WARN: 30
Remaining days which will create a CRIT: 15
You can change these default values and their created state in WATO.