app.js

var express = require('./server');
var app = express();

app.get('/cars/:id', function (req, res) {
  res.send({id: req.params.id, name: 'Corolla'});
}, {summary: 'Retrieve a car'});

var auth = function (req, res, next) {
  next();
};
auth.__meta__ = {
  auth: true
};

app.post('/cars', auth, function (req, res) {
  res.send(200);
}, {summary: 'Create a car'});

app.get('/', function (req, res) {
  res.send(app.describe());
}, {summary: 'Describe the API'});

app.listen(3000);

output.json

[
  {
    "method": "get",
    "path": "/cars/:id",
    "keys": [
      {
        "name": "id",
        "optional": false
      }
    ],
    "summary": "Retrieve a car"
  },
  {
    "method": "get",
    "path": "/",
    "keys": [],
    "summary": "Describe the API"
  },
  {
    "method": "post",
    "path": "/cars",
    "keys": [],
    "summary": "Create a car",
    "auth": true
  }
]

server.js

var express = require('express');
var app = express.application;
var methods = require('methods');
var _ = require('lodash');

exports = module.exports = express;

var meta = {};

methods.forEach(function (method) {
  meta[method] = {};

  var orig = app[method];
  app[method] = function(path) {
    var tmp = {};
    var lastArg = _.last(arguments);

    if (arguments.length >= 2 && _.isObject(lastArg)) {
      tmp = _.extend(tmp, lastArg);
      arguments = _.initial(arguments);
    }

    _.forEach(arguments, function (arg) {
      if (_.isFunction(arg) && _.has(arg, '__meta__')) {
         tmp = _.extend(tmp, arg.__meta__);
      }
    });

    meta[method][path] = tmp;

    return orig.apply(this, arguments);
  };
});

app.describe = function () {
  var endpoints = [];
  var map = this._router.map;

  var f = function (route) {
    endpoints.push(_.extend(_.pick(route, 'method', 'path', 'keys'), meta[route.method][route.path]));
  }.bind(this);

  map.forEach(function (method) {
    method.forEach(f);
  });

  return endpoints;
};