By Caroline Valetkevitch NEW YORK (Reuters) – The second half of the year started with gains in global stock indexes on Friday ahead of the long U.S. holiday weekend, while the 10-year Treasury yield fell the most since COVID-19 hit markets in March 2020. Copper prices slumped to their weakest in 17 months. Stocks were lower early in the New York session but rallied late to end higher. U.S. markets will be closed Monday for the U.S. Fourth of July holiday. “It’s a Friday before a long weekend, so market movements can be somewhat exaggerated,” said Peter Cardillo, chief market economist at Spar…