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

metrics/process_standard.go

@@ -0,0 +1,14 @@
+//go:build linux || windows
+
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
+)
+
+// registerProcessCollector registers the standard prometheus ProcessCollector
+// which uses procfs on Linux and Win32 syscalls on Windows.
+func registerProcessCollector(reg *prometheus.Registry) {
+	reg.MustRegister(collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}))
+}