fix(metrics): add cross-platform process metrics collector for non-Linux systems

The standard Prometheus ProcessCollector uses procfs (Linux-only /proc),
so process_resident_memory_bytes and process_virtual_memory_bytes were
silently absent on FreeBSD, Darwin, and other non-Linux platforms.

- Switch to a custom prometheus.Registry to isolate GOST metrics from
  the default registry auto-registrations
- Add gopsutil/v3-based process collector (process_other.go) for
  platforms without procfs (!linux && !windows)
- Keep standard ProcessCollector (process_standard.go) on Linux/Windows
- Update metrics service and handler/metrics to serve from
  promhttp.HandlerFor(customRegistry) instead of promhttp.Handler()
- Add Registry() accessor for the custom registry

Fixes go-gost/gost#509

handler/metrics/handler.go

@@ -43,7 +43,7 @@ func (h *metricsHandler) Init(md md.Metadata) (err error) {
 
 	xmetrics.Enable(true)
 
-	h.handler = promhttp.Handler()
+	h.handler = promhttp.HandlerFor(xmetrics.Registry(), promhttp.HandlerOpts{})
 
 	mux := http.NewServeMux()
 	mux.Handle(h.md.path, http.HandlerFunc(h.handleFunc))