The Boston GlobeAmid concerns the planet is warming, the market for clean, green technology is beginning to show signs of overheating, too. "I call it the global warming bubble,' said Bob Metcalfe, who knows something about bubbles. An inventor of the ethernet and founder of Marlborough, Massachussets networking company 3Com Corporation, the stock of which plummeted when the tech bubble burst in 2000, Metcalfe now serves as chief executive of Cambridge biofuel start-up Green Fuel Technologies Corporation.