If a metric definition is read-only, that means the definition can't be changed in a way that would invalidate metrics or metric samples recorded with it. Display-only values (such as captions and descriptions) can always be changed, and new metrics can always be added to a metric definition.
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