you can use ceilometer and grafana for its dashboard. and use aodh for alarming

Nagios will be a good option

You can use Zabbix monitoring tool, it is not the OpenStack tool, but it is used to monitoring your instance and you can also trigger a mail alert. Zabbix is an open source monitoring software tool for diverse IT components, including networks, servers, virtual machines (VMs) and cloud services. Zabbix provides monitoring metrics, such as network utilization, CPU load, and disk space consumption. The software monitors operations on Linux, Hewlett Packard Unix (HP-UX), Mac OS X, Solaris and other operating systems (OSes); however, Windows monitoring is only possible through agents

features of Zabbix Send messages, Let Zabbix fix issues automatically, Escalate problems according to flexible user-defined Service Levels, Customize messages based on recipient's role, Customize messages with runtime and inventory information.

For ubuntu follow this link for Zabbix configuration and adding the new host to Zabbix serverhttps://www.digitalocean.com/communit...

configure mail alerts on Zabbix serverhttps://www.oodlestechnologies.com/bl...

zabbix Documentationhttps://www.zabbix.com/documentation/...