The Google Analytics API only allows querying directly for measures and dimensions, and does not allow us to perform any aggregations on the data.
Many Google Analytics metrics are already aggregated in some way. For example, Sessions is already a count of sessions. Visitors is already a count of visitors.
If you would like to apply aggregations on top of Google Analytics metrics - such as average of sessions, you can do so in the Data Pipeline.
For definitions of individual metrics, see Google's documentation.