Simple express HTTP API request metrics gathering, demonstrating how `req.connection.bytesRead` is weird

request-metrics-gatherer.js

const express = require('express');
const bodyParser = require('body-parser');
const onFinished = require('on-finished');

// it may be necessary to modify the request object in various ways, this can
// be done in this function. For example, a `_metrics_gatherer` object is
// added to req and req.connection for the purposes of metrics observing functions
const modifyReq = (req) => {
    req._metrics_gatherer = {};
    if (!req.connection._metrics_gatherer) {
        req.connection._metrics_gatherer = {};
    }
}

// A specific sequence of steps is used to keep track of the changing values of
// req.connection.bytesRead and req.connection.bytesWritten
//
// These two quantities are observed when the request arrives and when it
// has finished to subtract the difference, rather than simply observing them
// when the request has finished, because the net.Socket objects (as
// `.connection` on Request objects) are re-used by express, and so
// connection.bytesRead will, at the very start of the request, give us the
// bytesRead/bytesWritten by the last request to use the same net.Socket object.
const observeBytesRW = (req, res) => {
    const bytesReadPreviously = req.connection._metrics_gatherer.bytesRead || 0;
    const bytesWrittenPreviously = req.connection._metrics_gatherer.bytesWritten || 0;
    return () => {
        const bytesReadDelta = req.connection.bytesRead - bytesReadPreviously;
        const bytesWrittenDelta = req.connection.bytesWritten - bytesWrittenPreviously;
        req.connection._metrics_gatherer.bytesRead = req.connection.bytesRead;
        req.connection._metrics_gatherer.bytesWritten = req.connection.bytesWritten;
        console.log(`bytesReadDelta: ${bytesReadDelta}`);
        console.log(`bytesWrittenDelta: ${bytesWrittenDelta}`);
    };
};

// observe the request latency using process.hrtime
const observeLatency = (req, res) => {
    const t0 = process.hrtime();
    return () => {
        const t1 = process.hrtime();
        const dt = (t1[0] - t0[0]) + (t1[1] - t0[1]) / 1e6;
        console.log(`dt: ${dt}`);
    };
};

// attach a middleware to all requests to observe various metrics
const metricsMiddleware = (req, res, next) => {
    modifyReq(req);
    const onFinishFuncs = [
        observeBytesRW(req, res),
        observeLatency(req, res)
    ];
    onFinished(res, () => {
        onFinishFuncs.forEach(f => f());
    });
    next();
};

const server = () => {
    const app = express();
    app.use(bodyParser.json());
    app.use(metricsMiddleware)
    app.all('*', (req, res) => {
        res.send(req.body);
    });
    const server = app.listen(process.env.PORT || 3001);
    console.log(`listening on ${process.env.PORT || 3001}`);
};

server();