What I don’t understand is how to get the “Reads, all pubkeys” type of stat. I mean the writes can be obtained from a firehose sub but I don’t get how to read a count of who reads from a relay. Specially a relay you don’t control
That's not "how often people open apps and read from relays". It's "how many people have this relay in their relay list with a 'read' marker" - meaning, how many people will use this relay to read events from it, as opposed to 'write' relays - those they will use to broadcast their events.