Fix exchanger stats GetPackets[Read,Written]() counts

Minor bug that these were returning number of *bytes* rather than number of packets.

PiperOrigin-RevId: 895495735

quiche/quic/qbone/bonnet/tun_device_packet_exchanger.h

diff --git a/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.h b/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.h
index 1f4d0fc..a277909 100644
--- a/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.h
+++ b/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.h
@@ -7,6 +7,8 @@
 
 #include <linux/if_ether.h>
 
+#include <cstddef>
+
 #include "quiche/quic/core/quic_packets.h"
 #include "quiche/quic/qbone/platform/kernel_interface.h"
 #include "quiche/quic/qbone/platform/netlink_interface.h"
@@ -28,8 +30,8 @@
 
     virtual ~StatsInterface() = default;
 
-    virtual void OnPacketRead(size_t count) = 0;
-    virtual void OnPacketWritten(size_t count) = 0;
+    virtual void OnPacketRead(size_t length) = 0;
+    virtual void OnPacketWritten(size_t length) = 0;
     virtual void OnReadError(std::string* error) = 0;
     virtual void OnWriteError(std::string* error) = 0;