Get statistics on the sales of Minecraft.

@param {Array<string>} [metricKeys] - list of metric keys to combine the stats of
@returns {Promise<{total: number, last24: number, saleVelocityPerSeconds: number}[]>} resolves if metricKeys is provided
@see {@link http://wiki.vg/Mojang_API#Statistics}
function getOrdersStatistics (metricKeys = ['item_sold_minecraft', 'prepaid_card_redeemed_minecraft']) {
  return fetch(`${CORE_API}/orders/statistics`, {
    method: 'POST',
    body: JSON.stringify({metricKeys}),
    headers: {
      'user-agent': USER_AGENT,
      'content-type': 'application/json',
      'accept': 'application/json'
    }
  })
    .then(handleErrors)
    .then(res => res.json())
}