By Niket Nishant and Manya Saini (Reuters) – For U.S. online trading companies an often-overlooked financial metric could act as a bulwark against softening demand in the third quarter, according to analysts. The U.S. Federal Reserve’s rate hikes are expected to drive up interest income for commission-free brokerage Robinhood Markets Inc and cryptocurrency exchange Coinbase Global Inc. “While rates were low and crypto markets were hot, investors paid little attention to (interest income), but it is becoming an increasingly important driver for Coinbase’s revenue stream,” Barclays analysts wrot…