Influxdb has been a real workhorse. We suffered through some of their early issues, but since then it's been extremely solid. It just runs, is space efficient, and very robust.
We almost went with Prometheus instead of Influx (as I said, early growing pains), but I had just struggled through managing a central inventory and hated it, so I really wanted a push rather than pull architecture. But from my limited playing with it, Prometheus seemed solid.