Prometheus is a monitoring system (it collects metrics and evaluates alerts on them), while TimescaleDB is a database. While it is possible to use TimescaleDB as a remote storage for Prometheus, I'd recommend taking a look at other remote storage integrations as well [1]. Some of them natively support PromQL (like VictoriaMetrics, m3db, Cortex), others may be easier to setup and operate. For instance, VictoriaMetrics works out of the box without complex configuration. Disclaimer: I work for VictoriaMetrics :).
[1] https://prometheus.io/docs/operating/integrations/#remote-en...