A good reasonably flexible system is smokeping. Debian packages
it. I use it to get ping and http/https request time for my
house machines and a few important outside machines.
If you have more significant future plans, it would be worth a
couple of hours to set up icinga2, a Nagios-compatible
monitoring system. It is also packaged by Debian.
(Nagios was the defacto open source standard, then the project
forked several ways as the main product went commercial.)
-dsr-