Currently Piwik_Archive_Single::getRequestedReportFor is (very indirectly) used to associate metric names w/ the plugins that process them. It employ three hacks, returns a 'report' name which contains the plugin name. ArchiveProcessing splits the report name and returns the plugin name. This is convoluted and the code is just a bunch of hacks so it should probably be changed.
I believe this was fixed in 2.0