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

service/service.go

@@ -302,6 +302,11 @@ func (s *defaultService) Close() error {
 	if closer, ok := s.handler.(io.Closer); ok {
 		closer.Close()
 	}
+	if s.options.observer != nil {
+		if closer, ok := s.options.observer.(io.Closer); ok {
+			closer.Close()
+		}
+	}
 	return s.listener.Close()
 }