Netdata is an open-source tool to collect real-time metrics. It's a growing product and can also be found on GitHub!
Apache HertzBeat
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Agentless real-time monitoring and alerting.
AppSignal
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APM tool for error tracking and performance monitoring.
Axiom
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Cloud-native architecture with built-in tracing and AI telemetry.
Baselime
Use logs, metrics, distributed tracing and wide-events to detect, diagnose and resolve issues.