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/config/v1/common.go

@@ -72,6 +72,26 @@ func (c *WebServerConfig) Complete() {
 	c.Addr = util.EmptyOr(c.Addr, "127.0.0.1")
 }
 
+// MetricsServerConfig configures a separate listener for health checks, metrics
+// (frps only), and optional pprof handlers. It does not serve the web interface.
+type MetricsServerConfig struct {
+	// Addr defaults to "127.0.0.1".
+	Addr string `json:"addr,omitempty"`
+	// Port disables the listener when set to 0.
+	Port int `json:"port,omitempty"`
+	// User and Password protect metrics and pprof; health checks remain anonymous.
+	User     string `json:"user,omitempty"`
+	Password string `json:"password,omitempty"`
+	// PprofEnable explicitly enables profiling, excluding cmdline.
+	PprofEnable bool `json:"pprofEnable,omitempty"`
+	// Enable TLS if TLS is not nil.
+	TLS *TLSConfig `json:"tls,omitempty"`
+}
+
+func (c *MetricsServerConfig) Complete() {
+	c.Addr = util.EmptyOr(c.Addr, "127.0.0.1")
+}
+
 type TLSConfig struct {
 	// CertFile specifies the path of the cert file that client will load.
 	CertFile string `json:"certFile,omitempty"`