When a billable metric has filters defined, set a default price on the charge level to cover all the other combination of filters not defined in the charge.
Add a charge to a plan (this charge should be associated with a billable metric containing filters);
Add a default price;
Set a price for this default price
Set a default price for a charge handling groups
If the default price is not set, Lago will automatically define it as 0 based on the charge model.