Point this at your modem's (internal, firewalled) IP address and enable the TELNET interface. It will log in and generate Prometheus metrics when scraped by running various diagnostic commands.
Basic usage:
./dsl320b-exporter --modem_ip 192.168.1.1 --modem_pass PASSWORD
So many. This is not polished code, by any stretch of the imagination.
The biggest one is that as yet there's no automatic reconnection of the telnet session, so if the exporter gets disconnected for any reason it will return bad data when scraped.
You may find your diagnostic output differs from mine, too!
See collectors.go. I'll probably write a list at some point when it changes less frequently.