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/client.go

@@ -38,6 +38,7 @@ func (v *ConfigValidator) ValidateClientCommonConfig(c *v1.ClientCommonConfig) (
 		func() (Warning, error) { return v.validateAuthConfig(&c.Auth) },
 		func() (Warning, error) { return nil, validateLogConfig(&c.Log) },
 		func() (Warning, error) { return nil, validateWebServerConfig(&c.WebServer) },
+		func() (Warning, error) { return nil, validateMetricsServerConfig(&c.MetricsServer, &c.WebServer) },
 		func() (Warning, error) { return validateTransportConfig(&c.Transport) },
 		func() (Warning, error) { return validateIncludeFiles(c.IncludeConfigFiles) },
 	}