Zabbix - Track, record, alert and visualize performance and availability of IT resources
InfluxData - Scalable datastore for metrics, events, and real-time analytics.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
KX - Kx Systems has a high-performance vector database, kdb+, with a built-in programming language, q, that sets the standard for time-series analytics.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Spotify Heroic - Heroic time series database.