PMM is your best choice when looking for an Open Source monitoring solution

Percona has a history of being good at providing really informative materials in form of blog post, documentation, webinars or conference presentations. T is also true to the Percona Monitoring and Management, you can find excellent source of information on their site or on the forum. Support tickets are also handled well.

I really like Grafana (the frontend of the PMM), but there are some minor things that sometimes don't click. For example the main dashboard lists all machines automatically, this page can be really slow when you have many instances(one can change the default landing page though). Sometimes the interface is not super responsive when making an overview for a lot of servers at once. Otherwise it's really well planned, logical and beautiful.

There are no other open source tools out there that could compete with PMM in its league. If you use MySQL (or PostgreSQL, ProxySQL) you can have a running monitoring in a couple of minutes. The agents are not really resource intensive and Percona provides a repo for most of the main Linux distributions. The documentation is thorough and straightforward. The defaults are good!