Note: Information provided from Windows 8 Developer Preview
Windows 8 has a new tab in the task manager called App History. This tab allows you to monitor the usage of Metro app.
With Windows 8 being designed for a more mobile workforce, I can see this tab being very useful. You can identify how much CPU time but more importantly, how much network bandwidth is being consumed. With our more mobile society, one network hogging app and really degrade the users experience.