Apache Benchmark - real concurrency level - issue - node.js

Question

Recently I've created simple node.js script to validate concurency of some long lasting DB operations. Basic idea of is to receive web request, than wait around 10 seconds and return result (code below).
/*jshint esversion: 6 */
const express = require('express');
const app = express();
var counter = 0;
app.get('/test', (req, res) => {
counter++;
console.log(counter);
// simulate long db query
setTimeout(() => {
res.send();
}, 10000);
});
app.listen(80, () => console.log('Example app listening on port 80!'));
Apache Benchmar tool was used to start initial test - by executing command with following parameters:
ab -c 20 -n 200 -v 3 http://localhost/test.
The problem is, that presented scripts shows only one !!! connection to be send by Apache Benchmark.
After deeper investigation I observed, that first request send by Apache Bechmark seems to be a kind of service status checker and is not executed concurrently. Request concurrency is enabled after first request. To illustrate this I prepared slightly modified version of the code, which just simply responds immediatelly to first request.
app.get('/test', (req, res) => {
counter++;
console.log(counter);
if (counter == 1) {
res.send();
return;
}
// simulate long db query
setTimeout(() => {
res.send();
}, 10000);
});
Modified version shows apache benchmark concurrency level to be as expected. Of course this testing approach is rather counter productive, do you know how to disable this unexpected bahaviour of Apache Benchmark ?