I was under the impression that influxdb's storage engine was going to be a viable solution for Prometheus at some point. Now they are writing their own; not sure where that interest went.
It's presently looking like Influx might once again be an option for long term storage for Prometheus.
I'm not sure how interested influxdb would be to the idea, given their shrewd moves towards monetization, but it would be nice if the storage engine could be developed as a component of influxdb and adopted into Prometheus(ala rocks, level, etc).