Prometheus - An open-source systems monitoring and alerting toolkit.
InfluxData - Scalable datastore for metrics, events, and real-time analytics.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
TimescaleDB - TimescaleDB is a time-series SQL database providing fast analytics, scalability, with automated data management on a proven storage engine.
StatsD - Simple daemon for easy stats aggregation.
Hosted Graphite - Graphite as a service