I tried this:
@Override
public Multimap<Attribute, AttributeModifier> getAttributeModifiers(EquipmentSlotType slot, ItemStack stack) {
Builder<Attribute, AttributeModifier> builder = ImmutableMultimap.builder();
builder.putAll(super.getAttributeModifiers(slot, stack));
builder.put(Attributes.ATTACK_DAMAGE, new AttributeModifier(BASE_ATTACK_DAMAGE_UUID, "Tool modifier", minedBlocks*0.2, AttributeModifier.Operation.ADDITION));
return builder.build();
}