How to pass request scope variables to promethues handler function - go

Question

i am building an promethues exporter in golang, the url to the exporter will be http://exporter-ip:9000/unique-id/metrics.
By parsing the url in ProcessParameters() function i am getting unique-id and with unique-id i am getting ip,username,password.
how can i pass IP, Username, Password from ProcessParameters() middleware function to Collect() function.
There variables are request scoped
func (collector *Collector) Collect(ch chan<- prometheus.Metric) {
//need IP,Username & Password here.
}
func ProcessParameters(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
log.Print("Executing middlewareOne")
DeviceID = strings.Split(r.URL.Path, "/")[1] //getting unique ID from the URL and verifying if that id is valid
_, ok := util.Devices[DeviceID] //if device id is not present in map, return StatusForbidden error in if block.
if !ok{
errMsg := "Device not found"
http.Error(w, errMsg, http.StatusForbidden)
log.Println(errMsg)
w.WriteHeader(http.StatusForbidden)
w.Header().Set("Content-Type", "application/json")
resp := make(map[string]string)
resp["message"] = "Forbidden"
jsonResp, err := json.Marshal(resp)
if err != nil {
log.Fatalf("Error happened in JSON marshal. Err: %s", err)
}
w.Write(jsonResp)
} else { //if id is present pass controller final handler(deviceHandler)
// tried setting it to request context also how to access it from collect() func
ctx := context.WithValue(r.Context(), "IP", util.Devices["10.0.0.1"])
context.WithValue(r.Context(), "UserName", util.Devices["user1"])
context.WithValue(r.Context(), "Password", util.Devices["pass1"])
next.ServeHTTP(w, r.WithContext(ctx))
}
})
}
func main() {
collector := metricsCollector()
registry := prometheus.NewRegistry()
registry.Register(collector)
deviceHandler := promhttp.HandlerFor(registry, promhttp.HandlerOpts{})
mux := http.NewServeMux()
mux.Handle("/", ProcessParameters(deviceHandler)) // how can pass variables from ProcessParameters() middleware handler to deviceHandler
err := http.ListenAndServe(":9090", mux)
log.Fatal(err)
}