Montgomery County Executive Marc Elrich is giving a tour of sorts around downtown Bethesda. It’s a mild, sunny day in late December, and he’s dressed in a leather jacket, playing equal parts chronicler and fortune teller of the pressures on affordable housing. As he points up and down Wisconsin Avenue, Elrich reels through a litany of planning terms: MPDUs (the “moderately priced dwelling units” the county requires in large development projects); affordable units (cheaper than MPDUs); and NOAH (the “naturally occurring affordable housing” he wants to protect).