The Key is the combination of metrics capture system label, category name, and counter name to uniquely identify a specific metric definition. It can also identify the same definition across different sessions.
Requirements
Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2