I'll certainly agree that this is an area where different opinions abound, and also you are much less likely to be prosecuted for this, so it's likely that advice would be that it's probably alright and you'll get away with it. But a strict interpretation of the law says that you can't use information gathered for a purpose for which the user didn't consent (or deliberately ask for, etc), even if you have it lying around because you collected it for a separate reason that is valid.

Regarding arguing that improving the user experience is a legitimate interest - I'm not aware of that having been argued and decided in court, but my opinion is that it is a hopeful misinterpretation of the law, and a slippery slope towards quite egregious data collection.

Yes, you can collect web site metrics without identifying information, for instance how many times are the different links on a particular page clicked on, but if you're linking one page request to another by the identity of the browser that is requesting them, then that is crossing the line.