Add walk attributes benchmark (#5547)

This adds a benchmark for `record.WalkAttributes`.

Part of #5054.

```
BenchmarkWalkAttributes/1_attributes-10                 346989372                3.449 ns/op           0 B/op          0 allocs/op
BenchmarkWalkAttributes/10_attributes-10                345712522                3.459 ns/op           0 B/op          0 allocs/op
BenchmarkWalkAttributes/100_attributes-10               349380534                3.455 ns/op           0 B/op          0 allocs/op
BenchmarkWalkAttributes/1000_attributes-10              342041373                3.484 ns/op           0 B/op          0 allocs/op
```

sdk/log/record_test.go

@@ -640,6 +640,35 @@ func TestTruncate(t *testing.T) {
 	}
 }
 
+func BenchmarkWalkAttributes(b *testing.B) {
+	for _, tt := range []struct {
+		attrCount int
+	}{
+		{attrCount: 1},
+		{attrCount: 10},
+		{attrCount: 100},
+		{attrCount: 1000},
+	} {
+		b.Run(fmt.Sprintf("%d attributes", tt.attrCount), func(b *testing.B) {
+			record := &Record{}
+			for i := 0; i < tt.attrCount; i++ {
+				record.SetAttributes(
+					log.String(fmt.Sprintf("key-%d", tt.attrCount), "value"),
+				)
+			}
+
+			b.ReportAllocs()
+			b.ResetTimer()
+
+			for i := 0; i < b.N; i++ {
+				record.WalkAttributes(func(log.KeyValue) bool {
+					return true
+				})
+			}
+		})
+	}
+}
+
 func BenchmarkSetAddAttributes(b *testing.B) {
 	kv := log.String("key", "value")