Split the set and add attributes benchmarks (#5546)

This benchmark currently tests two rather different methods within the
same loop, which makes it hard to see what could be causing a
performance degradation.

Related: #5054.

```
BenchmarkSetAddAttributes/SetAttributes-10              14066331                82.80 ns/op           48 B/op          1 allocs/op
BenchmarkSetAddAttributes/AddAttributes-10              19333711               114.7 ns/op             0 B/op          0 allocs/op
```

sdk/log/record_test.go

@@ -642,12 +642,24 @@ func TestTruncate(t *testing.T) {
 
 func BenchmarkSetAddAttributes(b *testing.B) {
 	kv := log.String("key", "value")
-	records := make([]Record, b.N)
 
-	b.ReportAllocs()
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		records[i].SetAttributes(kv)
-		records[i].AddAttributes(kv)
-	}
+	b.Run("SetAttributes", func(b *testing.B) {
+		records := make([]Record, b.N)
+
+		b.ResetTimer()
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			records[i].SetAttributes(kv)
+		}
+	})
+
+	b.Run("AddAttributes", func(b *testing.B) {
+		records := make([]Record, b.N)
+
+		b.ResetTimer()
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			records[i].AddAttributes(kv)
+		}
+	})
 }