app.js

const request = require("request");
const http = require("http");

var agent = new http.Agent({
    keepAlive: true,
    //maxSockets: 25,  // uncomment this line to fix the problem
    keepAliveMsecs: 3000
});

async function asyncGet() {
    await new Promise((resolve, reject) => {
        setTimeout(function() {
            request({
                agent: agent,
                url: "http://www.google.com",
                timeout: 5000,
            }, (error, response, body) => {
                if (!error) {
                    console.log(`request got ${response.statusCode}`);
                    resolve();
                } else {
                    console.log(`request got an error`, error);
                    reject();
                }
            });
        }, 10);
    });
}

async function awaitAll() {
    const promises = [...Array(500).keys()].map(() => asyncGet());
    await Promise.all(promises);
}

awaitAll();