With apologies to Marshall McLuhan, when it comes to advertising commerce, the medium isn't just the message. It's also the metrics. Or, perhaps it's the other way around: The metrics are the medium. It's a lesson I learned early in my career, as a young reporter covering media for Adweek nearly 30 years ago. It didn't take me long to realize that what people on Madison Avenue were actually buying during their "upfront" network TV negotiations wasn't TV shows -- or even commercial units placed within those TV programs. What they were buying -- and what they continue to …