Graphite[1] has interesting plugin ecosystem like Grafana[2] & has been around since 2009. Architecture of Open Source Applications book[3] has done a nice overview of the architecture. I really like how it reuses linux filesystem as data store, making archiving very simple.

[1]: https://graphiteapp.org/ [2]: https://grafana.com/ [3]: https://aosabook.org/en/v1/graphite.html