message: Read user registration metrics from database
  (fixes
diff: const Prometheus = require('prom-client')
  const orders = require('../data/mongodb').orders
  const challenges = require('../data/datacache').challenges
  -const users = require('../data/datacache').users
  const utils = require('../lib/utils')
  const config = require('config')
  +const models = require('../models')
  exports.serveMetrics = function serveMetrics (reg) {
  return (req, res, next) => {
  - utils.solveIf(challenges.exposedMetricsChallenge, () => (true))
  + utils.solveIf(challenges.exposedMetricsChallenge, () => { return true })
  res.set('Content-Type', reg.contentType)
  res.end(reg.metrics())
  }
  @@ -44,11 +44,13 @@ exports.observeMetrics = function observeMetrics () {
  register.registerMetric(userMetrics)
  const updateLoop = setInterval(() => {
  - orders.count({}).then(function (orders) {
  + orders.count({}).then(orders => {
  orderMetrics.set(orders)
  })
  + models.User.count().then(count => {
  + userMetrics.set(count)
  + })
  challengeMetrics.set(Object.keys(challenges).filter((key) => (challenges[key].solved)).length)
  - userMetrics.set(Object.keys(users).length)
  }, 5000)
  return {
  