Then don't send the correct source IP address, with simple statistics gathering like this I hardly expect they require a response. It would mean there would be no personal data whatsoever.
Most ISPs filter spoofed IP addresses nowadays[1]. Even if your ISP doesn't prevent it, NAT might. You wouldn't get a whole lot of responses this way, and there'd be a strong bias because of regional differences w.r.t. filtering.
You'd just send a UDP packet with a spoofed source address and forget about it. There's no need to open any sort of 2-way connection for data that's only being transmitted in one direction (from browser to metrics server).
Tor is blocked in some places and viewed as very suspicious in others. If you're already in a place where you're trying not to draw attention to yourself, using Tor might not a good option.
Wouldn't spoofing IP addresses risk the data packets being filtered out by ISPs or other upstream network providers? IMO if keeping IP addresses hidden is a concern, it'd be better to use something like TOR.
