[2796] compare with a constant instead of a magic number

src/bin/auth/statistics.cc.pre

@@ -142,7 +142,7 @@ Counters::incRequest(const MessageAttributes& msgattrs) {
         server_msg_counter_.inc(opcode_to_msgcounter[code]);
 
         // Opcode = 0: Query
-        if (code == 0) {
+        if (code == Opcode::QUERY_CODE) {
             // Recursion Desired bit
             if (msgattrs.requestHasRD()) {
                 server_msg_counter_.inc(MSG_QRYRECURSION);