fix: swap TAP rx/tx for correct VM perspective

TAP interface statistics are from the host's perspective:
- TAP rx_bytes = bytes host receives = bytes VM transmits
- TAP tx_bytes = bytes host transmits = bytes VM receives

The API documents these as "bytes received/transmitted by the VM",
so we need to swap them to match the VM's perspective.

Author: hiroTamada

lib/vm_metrics/collector.go

@@ -93,6 +93,9 @@ func ReadProcStatm(pid int) (rssBytes, vmsBytes uint64, err error) {
 
 // ReadTAPStats reads network statistics from a TAP device.
 // Reads from /sys/class/net/<tap>/statistics/{rx,tx}_bytes.
+// Note: Returns stats from host perspective. Caller must swap for VM perspective:
+// - rxBytes = host receives = VM transmits
+// - txBytes = host transmits = VM receives
 func ReadTAPStats(tapName string) (rxBytes, txBytes uint64, err error) {
 	basePath := filepath.Join("/sys/class/net", tapName, "statistics")
 

lib/vm_metrics/manager.go

@@ -89,8 +89,11 @@ func (m *Manager) GetInstanceStats(ctx context.Context, info InstanceInfo) *VMSt
 		if err != nil {
 			log.DebugContext(ctx, "failed to read TAP stats", "instance_id", info.ID, "tap", info.TAPDevice, "error", err)
 		} else {
-			stats.NetRxBytes = rxBytes
-			stats.NetTxBytes = txBytes
+			// TAP stats are from host perspective, swap for VM perspective:
+			// - TAP rx_bytes = host receives = VM transmits
+			// - TAP tx_bytes = host transmits = VM receives
+			stats.NetRxBytes = txBytes
+			stats.NetTxBytes = rxBytes
 		}
 	}