Include the client library if you want, but the wire format is ridiculously simple. I'll implement it from memory in a HN comment.

    http.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
        w.WriteHeader(http.StatusOK)
        w.Header().Add("content-type", "text/plain")
        w.Write([]byte("# HELP foo_bar The numbers of foos barred.\n# TYPE foo_bar counter\nfoo_bar 42\n"))
    })

The client library is largely to keep track of running counters (and gauges, histograms, etc.), with a small amount of code to actually report those metrics when scraped. It's a very simple format.