(Reuters) - Britain's troubled Metro Bank said Chief Executive Craig Donaldson had agreed with the board to step down at the end of the year and that Chief Transformation Officer Dan Frumkin will become interim head, effective Jan. 1. Metro said on Wednesday that Donaldson, who has held the post of CEO since 2009, had led the bank through what has been a challenging period and will remain available to the board as an advisor until the end of 2020. News of Donaldson's departure comes after..