Register with FQDN in URL

This replaces having the FQDN in the body in order to make client
certificate validation easier.

The proxy will continue to work with older clients as we still keep the
old endpoint. However any calls to `/poll/*` will use the FQDN provided
in the URL for registration.

cmd/client/main.go

@@ -174,7 +174,7 @@ func loop(c Coordinator, client *http.Client) error {
 		level.Error(c.logger).Log("msg", "Error parsing url:", "err", err)
 		return errors.Wrap(err, "error parsing url")
 	}
-	u, err := url.Parse("poll")
+	u, err := url.Parse("poll/" + *myFqdn)
 	if err != nil {
 		level.Error(c.logger).Log("msg", "Error parsing url:", "err", err)
 		return errors.Wrap(err, "error parsing url poll")

cmd/proxy/main.go

@@ -99,6 +99,7 @@ func newHTTPHandler(logger log.Logger, coordinator *Coordinator, mux *http.Serve
 	handlers := map[string]http.HandlerFunc{
 		"/push":    h.handlePush,
 		"/poll":    h.handlePoll,
+		"/poll/":   h.handlePollWithPath,
 		"/clients": h.handleListClients,
 		"/metrics": promhttp.Handler().ServeHTTP,
 	}
@@ -144,7 +145,17 @@ func (h *httpHandler) handlePush(w http.ResponseWriter, r *http.Request) {
 // handlePoll handles clients registering and asking for scrapes.
 func (h *httpHandler) handlePoll(w http.ResponseWriter, r *http.Request) {
 	fqdn, _ := ioutil.ReadAll(r.Body)
-	request, err := h.coordinator.WaitForScrapeInstruction(strings.TrimSpace(string(fqdn)))
+	h.pollWithFQDN(string(fqdn), w)
+}
+
+// handlePoll handles clients registering and asking for scrapes.
+func (h *httpHandler) handlePollWithPath(w http.ResponseWriter, r *http.Request) {
+	fqdn := r.URL.Path[len("/poll/"):]
+	h.pollWithFQDN(fqdn, w)
+}
+
+func (h *httpHandler) pollWithFQDN(fqdn string, w http.ResponseWriter) {
+	request, err := h.coordinator.WaitForScrapeInstruction(strings.TrimSpace(fqdn))
 	if err != nil {
 		level.Info(h.logger).Log("msg", "Error WaitForScrapeInstruction:", "err", err)
 		http.Error(w, fmt.Sprintf("Error WaitForScrapeInstruction: %s", err.Error()), 408)