历史费率值请求地址：
https://www.bitmex.com/api/v1/funding?&filter=%7B%22symbol%22%3A%20%22XBTUSD%22%7D&count=10&reverse=true
返回值fundingRate，最近10条历史费率值
当前费率请求地址：
https://www.bitmex.com/api/v1/instrument?filter=%7B%22rootSymbol%22%3A%20%22XBT%22%2C%22state%22%3A%20%22Open%22%7D&columns=fundingRate%2ClastPrice%2Cexpiry&count=6&reverse=false
返回值fundingRate，当前费率值