Reserve the specified numUnits for future use by this leaky bucket. The behavior is undefined unless 'unitsReserved() + unitsInBucket() + numUnits' can be represented by a 64‐bit unsigned integral type. Note that after this operation, this bucket may overflow. Also note that the time interval between the invocations of reserve and submitReserved or cancelReserved should be kept as short as possible; otherwise, the precision of the time interval calculated by calculateTimeToSubmit may be negatively affected.