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.
