Which metric monitoring tool supports time-series data and threshold-based alert routing?

A common tool that supports both time-series metric monitoring and threshold-based alert routing is Prometheus.

• Time-series data: Prometheus is built around collecting and storing metrics as time-series.
• Threshold-based alerts: It supports alerting rules via Alertmanager, which can route alerts based on conditions, labels, severity, and other criteria.

Other tools in this space include Grafana Loki/Prometheus stack, Datadog, and Zabbix, but if you mean an open-source metric monitoring tool, Prometheus + Alertmanager is the standard answer.