In addition to any and all powers heretofore granted to metropolitan utilities districts, any such district may, in its discretion, by authorization of its board of directors, contract to sell water for use by a waterworks and water distribution system owned and operated by a city of any class or village except a city of the metropolitan class. The water so sold shall be used for the same domestic, mechanical, public, and fire purposes as water which a metropolitan utilities district supplies the consumers served water directly by it. The rates for water so sold shall be fixed by the metropolitan utilities district, including therein a demand or capacity charge in addition to a charge for the volume of water delivered. All water so delivered shall be metered at its point of delivery. The cost of any main extensions necessary to deliver the water to the city or village contracting for such supply shall be paid by it and set forth in the contract. The term of such contract shall not exceed twenty-five years.