Except as otherwise provided by law, witness’ fees for each day’s actual attendance, when legally required to attend before a grand jury, are twelve dollars ($12) a day and mileage for each mile actually traveled in attendance as such witness, one way only twenty cents ($0.20). Such per diem and mileage are discretionary and shall only be allowed upon a showing that the allowances are necessary for the expenses of the witness in attending. The court may disallow any fees to a witness unnecessarily subpoenaed.