The answer would be: an increase in the production of new plasma HCO3-
In metabolic acidosis, the pH of the blood is too acid which means there is too much H+ ion in the blood. The kidney can compensate it by making more OH- and dump more H+ to the urine.
The H+ is not filtered, it was secreted by the cells. More H+ will decrease the pH of the urine, makes it more acid. Acid secretion allows more HCO3- production.
Bicarbonate or HCO3- is base ion and increasing its production will lower the blood pH.