Lengthening is much harder than shortening. The philtrum is skin and muscle bounded by the nose and the red lip, so the options are limited to either adding bulk or releasing and redistributing tissue, and both have honest trade-offs. Filler placed in the philtral columns and just above the lip can make it appear longer and fuller, though it is temporary. Fat grafting is longer lasting for some people but less predictable in this thin area. Surgical lengthening, such as a lower lip or Cupid's bow approach, is uncommon and carries a visible scar risk. In person I would also look at the nose, teeth and chin, since proportion often matters more than the single measurement. Please see a board-certified facial plastic surgeon who works on lips.