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/server.go

@@ -69,7 +69,8 @@ type ServerConfig struct {
 
 	SSHTunnelGateway SSHTunnelGateway `json:"sshTunnelGateway,omitempty"`
 
-	WebServer WebServerConfig `json:"webServer,omitempty"`
+	WebServer     WebServerConfig     `json:"webServer,omitempty"`
+	MetricsServer MetricsServerConfig `json:"metricsServer,omitempty"`
 	// EnablePrometheus will export prometheus metrics on webserver address
 	// in /metrics api.
 	EnablePrometheus bool `json:"enablePrometheus,omitempty"`
@@ -105,6 +106,7 @@ func (c *ServerConfig) Complete() error {
 	c.Log.Complete()
 	c.Transport.Complete()
 	c.WebServer.Complete()
+	c.MetricsServer.Complete()
 	c.SSHTunnelGateway.Complete()
 
 	c.BindAddr = util.EmptyOr(c.BindAddr, "0.0.0.0")