The solution is just to
if value < max_value:
value = clamp(value + 1, 0, max_value)
$Label.text = "ST " + str(value) + " / " + str(max_value)
I do think you need to reconsider your
if conditional that I mentioned in the comment above - I don't think it's doing what you think it's doing.
Global.damage * delta will be a small number since
delta is usually very small (think > 0.2) so
value -= Global.damage could end up negative.