curl --request GET \
  --url http://localhost:8080/promo-groups/{group_id}

{
  "id": 123,
  "name": "<string>",
  "server_discount_percent": 123,
  "traffic_discount_percent": 123,
  "device_discount_percent": 123,
  "apply_discounts_to_addons": true,
  "is_default": true,
  "period_discounts": {},
  "auto_assign_total_spent_kopeks": 123,
  "members_count": 0,
  "created_at": "2023-11-07T05:31:56Z",
  "updated_at": "2023-11-07T05:31:56Z"
}