Curious why hyperloglog was used for this? It doesn't seem like something that would require a sophisticated approach. Also, why no mutex, is this thread safe, because in a web server it's going to be called from goroutines? Have you tested it in a real server with -race?
They probably at least need a mutex round the counter map, I don't see anything in hyperloglog either, so not sure this would be safe to use across goroutines (which is built in to the net/http server). Relatively easy to fix though.