https://travis-ci.org/github/bitcoin/bitcoin/jobs/680452476#L6107
"The request target will be clamped between 2 and the highest target\n"
            "fee estimation is able to return based on how long it has been running.\n"
            "An error is returned if not enough transactions and blocks\n"
            "have been observed to make an estimate for any number of blocks."},
                    }},
                RPCExamples{
                    HelpExampleCli("estimatesmartfee", "6")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    RPCTypeCheck(request.params, {UniValue::VNUM, UniValue::VSTR});
    RPCTypeCheckArgument(request.params[0], UniValue::VNUM);
    unsigned int max_target = ::feeEstimator.HighestTargetTracked(FeeEstimateHorizon::LONG_HALFLIFE);
    unsigned int conf_target = ParseConfirmTarget(request.params[0], max_target);
    bool conservative = true;
    if (!request.params[1].isNull()) {
        FeeEstimateMode fee_mode;
        if (!FeeModeFromString(request.params[1].get_str(), fee_mode)) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid estimate_mode parameter");
        }
        if (fee_mode == FeeEstimateMode::ECONOMICAL) conservative = false;
    }
    UniValue result(UniValue::VOBJ);
    UniValue errors(UniValue::VARR);
    FeeCalculation feeCalc;
    CFeeRate feeRate = ::feeEstimator.estimateSmartFee(conf_target, &feeCalc, conservative);
    if (feeRate != CFeeRate(0)) {
        result.pushKV("feerate", ValueFromAmount(feeRate.GetFeePerK()));
    } else {
        errors.push_back("Insufficient data or no feerate found");