Update (Jul 30, 2019): clarified the sunset schedule.
Starting the week of September 30, 2019, the 
average position metric will be sunset. This change was announced this past February in the 
Google Ads Help Center. The new alternatives were described in this November 2018 
blog post. Once the sunset occurs, requests for the average position (
AdWords API, 
Google Ads API) will return NULL values.
In the AdWords API v201809, NULL values are returned as 
two dashes. In the Google Ads API, NULL values are returned as 
NullValues.
If you use 
ValueTrack parameters, we encourage you to remove the ValueTrack parameter {adposition} from your tracking templates. When average position is sunset, the ValueTrack parameter {adposition} will begin returning the empty string.
Note that average position has already been removed from the current 
beta version of the Google Ads API, but is accessible in the earlier V1_x versions.
In lieu of average position, we recommend you use the new metrics of 
Impression (Absolute Top) %" and "Impression (Top) %. These are the new metrics:
As always, don't hesitate to reach out to us on 
the developer forum with any questions.
David Wihl, Google Ads API Team