Unless the rights, powers, functions, and obligations of a metropolitan municipal corporation have been assumed by a county as provided in chapter 36.56 RCW, a metropolitan municipal corporation shall be governed by a metropolitan council composed of elected officials of the component counties and component cities, and possibly other persons, as determined by agreement of each of the component counties and the component cities equal in number to at least twenty-five percent of the total number of component cities that have at least seventy-five percent of the combined component city populations. The agreement shall remain in effect until altered in the same manner as the initial composition is determined.