hotp.go
func (otp HOTP) Counter() uint64 {
	counter := binary.BigEndian.Uint64(otp.counter[:])
	return counter
}
func (otp HOTP) Counter() uint64 {
	counter := binary.BigEndian.Uint64(otp.counter[:])
	return counter
}
// Counter returns the HOTP's 8-byte counter as an unsigned 64-bit
// integer.