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.