code: function responseTime (options) {
    var opts = options || {}
  
    if (typeof options === 'number') {
      // back-compat single number argument
      deprecate('number argument: use {digits: ' + JSON.stringify(options) + '} instead')
      opts = { digits: options }
    }
  
    // get the function to invoke
    var fn = typeof opts !== 'function'
      ? createSetHeader(opts)
      : opts
  
    return function responseTime (req, res, next) {
      var startAt = process.hrtime()
  
      onHeaders(res, function onHeaders () {
        var diff = process.hrtime(startAt)
        var time = diff[0] * 1e3 + diff[1] * 1e-6
  
        fn(req, res, time)
      })
  
      next()
    }
  }
docs: Create a middleware to add a `X-Response-Time` header displaying
  the response duration in milliseconds.
  
  @param {object|function} [options]
  @param {number} [options.digits=3]
  @param {string} [options.header=X-Response-Time]
  @param {boolean} [options.suffix=true]
  @return {function}
  @public