OP was only asking about parking over ONE night. According to the Metro website "All-day parking, up to 24 hours, is available at all stations that have Metro-operated parking lots." i.e. Overnight parking (not to exceed 24 hours total) is perfectly proper at any Metro-operated lot.

Obviously, they won't know exactly when you parked, so the "24 hours" can only be loosely enforce. Just don't park over 2 nights - that might be asking for enforcement of the 24-hour rule.