The Key is the combination of metrics capture system label, category name, and counter name of the metric definition, along with the instance name, to uniquely identify a specific metric instance of a specific metric definition. It can also identify the same metric instance 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