I've done a mediation SDK for DFP to put it in AdMob mediation (and viceversa).
For the moment, it works in dfp with SDK Mediation Creative (shows me an ad, and dfp counts impressions and clicks) but, in LOGCAT I can see this:
11-26 17:55:19.289: I/Ads(3413): Starting ad request.
11-26 17:55:21.309: I/Ads(3413): Trying mediation network: 1.8351550913290783E19
11-26 17:55:21.319: I/Ads(3413): Instantiating mediation adapter: com.google.ads.mediation.customevent.CustomEventAdapter
11-26 17:55:21.354: I/Ads(3413): Starting ad request.
11-26 17:55:22.614: I/Ads(3413): Scheduling ad refresh 30000 milliseconds from now.
11-26 17:55:22.619: I/Ads(3413): Ad finished loading.
11-26 17:55:22.624: I/Ads(3413): Ad finished loading.
11-26 17:55:52.634: I/Ads(3413): Starting ad request.
11-26 17:55:53.939: I/Ads(3413): Scheduling ad refresh 30000 milliseconds from now.
11-26 17:55:53.944: I/Ads(3413): Ad finished loading.
11-26 17:55:53.954: W/Ads(3413): Mediation adapter com.google.ads.mediation.customevent.CustomEventAdapter refreshed, but mediation adapters should never refresh.
11-26 17:55:53.954: I/Ads(3413): Ad finished loading.