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...

[4] https://github.com/prometheus/prometheus/issues/3746