fix(agent): use nvethernet MAC counters (#2090)

Co-authored-by: hank <hank@henrygd.me>

agent/network_counters_linux_test.go

@@ -0,0 +1,53 @@
+//go:build linux && testing
+
+package agent
+
+import (
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEthtoolCounterCombinesHighWord(t *testing.T) {
+	stats := map[string]uint64{
+		"counter":   123,
+		"counter_h": 2,
+	}
+
+	value, ok := ethtoolCounter(stats, "counter", "counter_h")
+	require.True(t, ok)
+	assert.Equal(t, uint64(2<<32+123), value)
+}
+
+func TestEthtoolCounterAddsLowWordAbove32Bits(t *testing.T) {
+	// nvethernet accumulates each register in 64-bit software fields, so the low
+	// word can carry past 32 bits; OR would drop the overlapping bit.
+	stats := map[string]uint64{
+		"counter":   1<<32 + 5,
+		"counter_h": 1,
+	}
+
+	value, ok := ethtoolCounter(stats, "counter", "counter_h")
+	require.True(t, ok)
+	assert.Equal(t, uint64(2<<32+5), value)
+}
+
+func TestEthtoolCounterRejectsOverflow(t *testing.T) {
+	stats := map[string]uint64{
+		"counter":   math.MaxUint64,
+		"counter_h": 1,
+	}
+
+	_, ok := ethtoolCounter(stats, "counter", "counter_h")
+	assert.False(t, ok)
+}
+
+func TestEthtoolCounterFallsBackToLowWord(t *testing.T) {
+	stats := map[string]uint64{"counter": 456}
+
+	value, ok := ethtoolCounter(stats, "counter", "counter_h")
+	require.True(t, ok)
+	assert.Equal(t, uint64(456), value)
+}