usermetric: add initial user-facing metrics

This commit adds a new usermetric package and wires
up metrics across the tailscale client.

Updates tailscale/corp#22075

Co-authored-by: Anton Tolchanov <anton@tailscale.com>
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

client/tailscale/localclient.go

@@ -353,6 +353,12 @@ func (lc *LocalClient) DaemonMetrics(ctx context.Context) ([]byte, error) {
 	return lc.get200(ctx, "/localapi/v0/metrics")
 }
 
+// UserMetrics returns the user metrics in
+// the Prometheus text exposition format.
+func (lc *LocalClient) UserMetrics(ctx context.Context) ([]byte, error) {
+	return lc.get200(ctx, "/localapi/v0/usermetrics")
+}
+
 // IncrementCounter increments the value of a Tailscale daemon's counter
 // metric by the given delta. If the metric has yet to exist, a new counter
 // metric is created and initialized to delta.

client/web/web.go

@@ -283,6 +283,12 @@ func (s *Server) serve(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	if r.URL.Path == "/metrics" {
+		r.URL.Path = "/api/local/v0/usermetrics"
+		s.proxyRequestToLocalAPI(w, r)
+		return
+	}
+
 	if strings.HasPrefix(r.URL.Path, "/api/") {
 		switch {
 		case r.URL.Path == "/api/auth" && r.Method == httpm.GET: