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/validation/server.go

@@ -44,6 +44,9 @@ func (v *ConfigValidator) ValidateServerConfig(c *v1.ServerConfig) (Warning, err
 	if err := validateWebServerConfig(&c.WebServer); err != nil {
 		errs = AppendError(errs, err)
 	}
+	if err := validateMetricsServerConfig(&c.MetricsServer, &c.WebServer); err != nil {
+		errs = AppendError(errs, err)
+	}
 	if c.Transport.TLS.UseSystemCa {
 		errs = AppendError(errs, fmt.Errorf("transport.tls.useSystemCa is only supported in frpc transport.tls"))
 	}