Here’s an example of the configuration:

[http_loadtime]
     env.target https://pydelion.com

Enabling:

ln -s /usr/share/munin/plugins/http_loadtime /etc/munin/plugins/http_loadtime

Test run:

[root@node1 ~]# sudo munin-run http_loadtime
loadtime.value 1.04
[root@node1 ~]#

Munin restart:

[root@node1 ~]# service  munin-node restart
Stopping Munin Node agents:                                [  OK  ]
Starting Munin Node:                                       [  OK  ]
[root@node1 ~]#

If you need external check you can use pingdom tools, for instance.