public byte messageType() {
        return isOneWay() ? TMessageType.ONEWAY : TMessageType.CALL;
    }
Returns the type of this function.

@return {@link TMessageType#CALL} or {@link TMessageType#ONEWAY}
thrift/thrift0.13/src/main/java/com/linecorp/armeria/internal/common/thrift/ThriftFunction.java