+1 for hetrix (basic monitoring & uptime page for customers)
+1 for netdata (free/affordable with a lot of detail and plays nice with docker & cgroups. Shows resource usage per site
hosted thanks to this.
Netdata also sends useful alerts out of the box. For example if the disk is rapidly filling up, it will send warning email "at this rate out of disk space will happen in x time." They now have an app that is the ugliest UX but sends notifications to your phone.
Longview is okay if you're on linode for basic resource usage monitoring but its not realtime (even if they say it is)