curl --request GET \
  --url http://localhost:8080/cabinet/promo/loyalty-tiers

{
  "tiers": [
    {
      "id": 123,
      "name": "<string>",
      "threshold_rubles": 123,
      "server_discount_percent": 0,
      "traffic_discount_percent": 0,
      "device_discount_percent": 0,
      "period_discounts": {},
      "is_current": false,
      "is_achieved": false
    }
  ],
  "current_spent_rubles": 123,
  "current_tier_name": "<string>",
  "next_tier_name": "<string>",
  "next_tier_threshold_rubles": 123,
  "progress_percent": 0
}