Investors need to keep an eye on this key profitability metric.When digging through financial statements, it's important to look at the trends for metrics rather than just a single quarter of results. This can lead to a better picture of a company's performance, and I've pinpointed one for Nvidia (NVDA -1.66%) that investors should watch.The trend of this metric is concerning and could cause weakness in the stock.Unprecedented demand for Nvidia's GPUs has raised profit levelsNvidia's graphics processing uni ...