Error:conectar ETIMEDOUT al raspar

Como se discutió en los comentarios, si desea controlar la cantidad máxima de solicitudes que están en curso al mismo tiempo, puede usar Bluebird para hacerlo con bastante facilidad de esta manera:

const Promise = require('bluebird');
const rp = require('request-promise');

router.get('/', (req, res) => {

    let collection = db.get().collection('foo');
    let collection2 = db.get().collection('test');
    collection.distinct('id', (err, idArr) => { // count: 3000+
        if (err) {
            // handle error here, send some error response
            res.status(501).send(...);
        } else {
            Promise.map(idArr, id => {
                let url = 'https://externalapi.io/id=' + id
                return rp(url).then(body => {
                    if (error) {
                        console.log(error)
                    } else {
                        let resp = JSON.parse(body);
                        // probably want to return a promise here too, but I'm unsure what DB you're using
                        collection2.insert(resp);
                    }
                }).catch(err => {
                    // decide what you want to do when a single request fails here
                    // by providing a catch handler that does not rethrow, other requests will continue
                });
                   // pick some concurrency value here that does not cause errors
            }, {concurrency: 10}).then(() => {
                // all requests are done, send final response
                res.send(...);
            }).catch(err => {
                // your code may never get here (depends upon earlier .catch() handler)
            });
        }
    });
});