InfluxDB is open source ( https://github.com/influxdata ) and seems to be leader by far as per DBEngines ranking : https://db-engines.com/en/ranking/time+series+dbms
I've been using Influx v1 on a small scaleway instance for a while.
I loved it and the tooling around it as well.
I would get notifications on Telegram if events needing attention happened.
But v3 is still under development so I'll explore Prometheus.