feat: add metricsServer for health checks, metrics and pprof (#5578)

Add a separate HTTP listener for observability, configured through
metricsServer in both frps and frpc. It serves /healthz and, when
pprofEnable is set, the pprof handlers except /debug/pprof/cmdline. It
never serves the dashboard, static assets or the admin API, and it does
not touch the global assets filesystem.

frps also serves /metrics on this listener with both runtime and business
metrics, enabled by metricsServer.port alone. The in-memory collector
still starts only with the dashboard, and the Prometheus collector starts
either with metricsServer or with enablePrometheus plus webServer, so a
configuration that collected nothing before still collects nothing.

The /healthz, /metrics and pprof endpoints on webServer, and
enablePrometheus, keep working but are deprecated and log a hint at
startup. frpc does not serve /metrics yet.

pkg/util/http/server.go

@@ -48,7 +48,18 @@ type Server struct {
 func NewServer(cfg v1.WebServerConfig) (*Server, error) {
 	assets.Load(cfg.AssetsDir)
 
-	addr := net.JoinHostPort(cfg.Addr, strconv.Itoa(cfg.Port))
+	s, err := newServer(cfg.Addr, cfg.Port, cfg.User, cfg.Password, cfg.TLS)
+	if err != nil {
+		return nil, err
+	}
+	if cfg.PprofEnable {
+		s.registerPprofHandlers()
+	}
+	return s, nil
+}
+
+func newServer(bindAddr string, port int, user, password string, tlsConfig *v1.TLSConfig) (*Server, error) {
+	addr := net.JoinHostPort(bindAddr, strconv.Itoa(port))
 	if addr == ":" {
 		addr = ":http"
 	}
@@ -65,11 +76,8 @@ func NewServer(cfg v1.WebServerConfig) (*Server, error) {
 		hs:     hs,
 		router: router,
 	}
-	if cfg.PprofEnable {
-		s.registerPprofHandlers()
-	}
-	if cfg.TLS != nil {
-		cert, err := tls.LoadX509KeyPair(cfg.TLS.CertFile, cfg.TLS.KeyFile)
+	if tlsConfig != nil {
+		cert, err := tls.LoadX509KeyPair(tlsConfig.CertFile, tlsConfig.KeyFile)
 		if err != nil {
 			return nil, err
 		}
@@ -77,7 +85,7 @@ func NewServer(cfg v1.WebServerConfig) (*Server, error) {
 			Certificates: []tls.Certificate{cert},
 		}
 	}
-	s.authMiddleware = netpkg.NewHTTPAuthMiddleware(cfg.User, cfg.Password).SetAuthFailDelay(200 * time.Millisecond).Middleware
+	s.authMiddleware = netpkg.NewHTTPAuthMiddleware(user, password).SetAuthFailDelay(200 * time.Millisecond).Middleware
 
 	// Bind only after all fallible preparation so an error cannot leak the listener.
 	ln, err := net.Listen("tcp", addr)