input: [Description] The number of times that the code for a pre-fetch ad tag was served by an ad server to a user's browser. This number can be greater than or equal to the number of impressions that are delivered. This is because, in some cases, the code for a pre-fetch ad is served to a user's browser but the ad isn't displayed. As a result, the code can be served without an impression's being counted. [Metric] 
output: Rich Media pre-fetch serves