Top Netflix executives who gathered for its annual business review this spring had a lot to be cheerful about. Profits were rising, customer defections remained at industry lows and it had hit franchises including “Bridgerton” and “Stranger Things.” But one metric was pointing in the wrong direction: Subscriber engagement was showing signs of decline, according to attendees. At the time, it was a small part of a conversation about the company’s goals, but it has since become a frequent topic of discussion at meetings, people familiar with the matter said. Engagement, which measures how long people spend watching content and…