The technical details of their software are described in https://eng.uber.com/m3/
This looks like a competitor to Cortex (https://www.cncf.io/blog/2018/12/18/cortex-a-multi-tenant-ho...).
They took a different path on the ”never build your own database” question.
Another viable player that took the path similar to M3DB is VictoriaMetrics [1]. This allowed implementing various features [2] and optimizations [3] without the need to negotiate their integration into upstream Prometheus. Such negotiations can stuck forever. [4]
[1] https://github.com/VictoriaMetrics/VictoriaMetrics/
[2] https://github.com/VictoriaMetrics/VictoriaMetrics/wiki/Exte...
[3] https://medium.com/@valyala/measuring-vertical-scalability-f...