fix(observer): add nil guards, fix resource leak, silent event drops, and missing doc comments

- WrapUDPConn, WrapPacketConn, WrapListener: add nil guards for pc/ln params
- NewGRPCPlugin: return no-op observer instead of nil on connection error
- gRPC plugin: fix resource leak by wiring Close() through observerWrapper
  and service.Close() lifecycle
- Both plugins: log unknown event types instead of silently dropping them
- httpPlugin: remove dead nil check for always-non-nil client field
- Stats.Reset: update core interface doc to match implementation behavior
- Add doc comments to all exported symbols across the observer package

registry/observer.go

@@ -2,6 +2,7 @@ package registry
 
 import (
 	"context"
+	"io"
 
 	"github.com/go-gost/core/observer"
 )
@@ -37,3 +38,15 @@ func (w *observerWrapper) Observe(ctx context.Context, events []observer.Event,
 	}
 	return v.Observe(ctx, events, opts...)
 }
+
+// Close closes the underlying observer if it implements io.Closer.
+func (w *observerWrapper) Close() error {
+	v := w.r.get(w.name)
+	if v == nil {
+		return nil
+	}
+	if closer, ok := v.(io.Closer); ok {
+		return closer.Close()
+	}
+	return nil
+}