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
Previously InfluxDB was Open Core, with very crippled Community version (ie High Availability was Enterprise only)
I’ve personally found TimescaleDB to be a much easier to work with platform, even if there is more overhead in defining a schema and enabling compression, etc.
But v3 is still under development so I'll explore Prometheus.