cli/serve: funnel interactive enablement flow tweaks

1. Add metrics to funnel flow.
2. Stop blocking users from turning off funnels when no longer in
   their node capabilities.
3. Rename LocalClient.IncrementMetric to IncrementCounter to better
   callout its usage is only for counter clientmetrics.

Updates tailscale/corp#10577

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

cmd/tailscale/cli/funnel.go

@@ -94,8 +94,13 @@ func (e *serveEnv) runFunnel(ctx context.Context, args []string) error {
 	}
 	port := uint16(port64)
 
-	if err := e.verifyFunnelEnabled(ctx, st, port); err != nil {
-		return err
+	if on {
+		// Don't block from turning off existing Funnel if
+		// network configuration/capabilities have changed.
+		// Only block from starting new Funnels.
+		if err := e.verifyFunnelEnabled(ctx, st, port); err != nil {
+			return err
+		}
 	}
 
 	dnsName := strings.TrimSuffix(st.Self.DNSName, ".")
@@ -176,7 +181,7 @@ func printFunnelWarning(sc *ipn.ServeConfig) {
 		p, _ := strconv.ParseUint(portStr, 10, 16)
 		if _, ok := sc.TCP[uint16(p)]; !ok {
 			warn = true
-			fmt.Fprintf(os.Stderr, "Warning: funnel=on for %s, but no serve config\n", hp)
+			fmt.Fprintf(os.Stderr, "\nWarning: funnel=on for %s, but no serve config\n", hp)
 		}
 	}
 	if warn {