Most of the time the client does have that info, in the form of logs, and usually games that have combat logs, are the easiest to make DPS meter’s with. But here’s one that collects packets:
Used for TERA. I’m sure regardless of sending over fake data, there is always a way to gather the info required. An example might be the numbers popping up on your screen when you hit things, if that info can be gathered in some way, a DPS meter can be made. I don’t want to get technical, since I don’t have the expertise to back it up, but I’ve seen plenty of creative ways for people to make DPS meters.
On another note, regardless of a DPS meter, people will still discriminate. Tier lists will exist, and it’s already happening. Even in the CBT, people did not want wizards without linkers, or clerics that weren’t priest, and it wasn’t because of some DPS meter, it was because of efficiency. Eventually people will know what does that super high damage, and what doesn’t, and it won’t be a DPS meter that tells people that.