Proxmox is an excellent open-source virtualization platform that integrates the KVM hypervisor and lightweight LXC containers. Besides the built-in web interface it offers a feature complete API to automate management and provisioning. Of course the API also provides a good way to collect cluster, hosts and guests metrics.
Zabbix is an open-source enterprise-level software designed for real-time monitoring of millions of metrics collected from tens of thousands of servers, virtual machines and network devices. Besides a powerful agent, SNMP, JMX and IPMI interfaces it offers the zabbix_sender utility which makes it very easy to send data from the command line or scripts.
For a recent project we created a Proxmox cluster monitoring integration for Zabbix. It features low level discovery of cluster nodes and collects quorum and nodes status, RAM/CPU usage, KSM sharing, vRAM allocation and usage, vCPU and vHDD allocations, number of VMs and LXC containers running or stopped.
This solution is made available as free software (GPL3.0) on GitHub.