Monitor your Server for Free!

gzDenis

A free tool that's very popular and easy to install, in order to monitor your server, is available as open source.
Netdata is probably the best tool you can find to monitor your server(s)
Installation
bash <(curl -Ss https://my-netdata.io/kickstart.sh)
Once installed, you will be able to see servers stats at http://your-ip:19999

gzDenis

You can dig further on modifying and work with netdata, your basic functions are:

systemctl stop netdata
systemctl start netdata
systemctl restart netdata