Prometheus - An open-source systems monitoring and alerting toolkit.
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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
Fluent Bit - Data collector and log forwarder.
collectd - System and applications metrics collector