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/client.go

@@ -60,10 +60,11 @@ type ClientCommonConfig struct {
 	// set.
 	Start []string `json:"start,omitempty"`
 
-	Log        LogConfig             `json:"log,omitempty"`
-	WebServer  WebServerConfig       `json:"webServer,omitempty"`
-	Transport  ClientTransportConfig `json:"transport,omitempty"`
-	VirtualNet VirtualNetConfig      `json:"virtualNet,omitempty"`
+	Log           LogConfig             `json:"log,omitempty"`
+	WebServer     WebServerConfig       `json:"webServer,omitempty"`
+	MetricsServer MetricsServerConfig   `json:"metricsServer,omitempty"`
+	Transport     ClientTransportConfig `json:"transport,omitempty"`
+	VirtualNet    VirtualNetConfig      `json:"virtualNet,omitempty"`
 
 	// FeatureGates specifies a set of feature gates to enable or disable.
 	// This can be used to enable alpha/beta features or disable default features.
@@ -94,6 +95,7 @@ func (c *ClientCommonConfig) Complete() error {
 	c.Log.Complete()
 	c.Transport.Complete()
 	c.WebServer.Complete()
+	c.MetricsServer.Complete()
 
 	c.UDPPacketSize = util.EmptyOr(c.UDPPacketSize, 1500)
 	return nil