GODRIVER-2542 optimize objectid hex func (#1062)

Co-authored-by: Qingyang Hu <[email protected]>

Author: isopov

bson/primitive/objectid.go

@@ -61,7 +61,9 @@ func (id ObjectID) Timestamp() time.Time {
 
 // Hex returns the hex encoding of the ObjectID as a string.
 func (id ObjectID) Hex() string {
-	return hex.EncodeToString(id[:])
+	var buf [24]byte
+	hex.Encode(buf[:], id[:])
+	return string(buf[:])
 }
 
 func (id ObjectID) String() string {

bson/primitive/objectid_test.go

@@ -28,6 +28,13 @@ func TestString(t *testing.T) {
 	require.Contains(t, id.String(), id.Hex())
 }
 
+func BenchmarkHex(b *testing.B) {
+	id := NewObjectID()
+	for i := 0; i < b.N; i++ {
+		id.Hex()
+	}
+}
+
 func TestFromHex_RoundTrip(t *testing.T) {
 	before := NewObjectID()
 	after, err := ObjectIDFromHex(before.Hex())