WASHINGTON (Reuters) - U.S. consumer credit in December grew by the most in nearly a year due to a sharp increase in credit card usage, a potentially positive sign for the economy. Total consumer credit rose by $18.8 billion to $3.1 trillion, the Federal Reserve said on Friday. That was the biggest gain since February. Economists polled by Reuters had expected consumer credit to rise by $12 billion in December. Revolving credit, which mostly measures credit-card use, rose by $5 billion in December after climbing $465 million in November. Revolving credit figures can be volatile. Non-revolving credit, which includes auto loans as well as student loans made by the government, increased $13.8 billion in December. (Reporting by Jason Lange; Editing by Paul Simao)