DFP counts the following metrics:
The ads served by DFP are all rendered inside an
<iframe>, which allows the use of creative wrappers. These are snippets of code that are wrapped around an advert upon rendering, and can be used to serve third-party tracking pixels, which aid in measuring advert viewability.
This is used by ads, and the third-party responsible is MOAT, a Media Rating Council accredited real-time analytics service.
For greater detail see:
DFP Counting impressions and clicks
DFP Ad server reporting metrics
Adding creative wrappers