disclaimer: I haven’t try it out yet. planning to move from heroku to here. 
just doing a POC now.
I have read this instrumentation (Metrics on Fly · Fly Docs) and it said that we can have custom metrics on my own apps. I don’t have much, it’s around 100-ish metrics.
I want to know pricing plan for Prometheus? if it’s 100% free, how long are logs kept? or the safest way to permanently keep those data is having my own self-hosted prometheus and loki 
Same question asking here but no-one answer (Logging retention policies).