There’s a metaphor in Reverend Al Sharpton’s new MSNBC “Lean Forward” commercial — something about growing up in Brooklyn, blueberry pie, Republicans, and the economy — but it’s a little muddied, not unlike his show on the network, PoliticsNation. But Sharpton gets charmingly worked up while standing on that stoop, and we get the idea. Plus, if there were ever a week for pie talk …