if field 1 a multi-value field? or is that representing the value of field1 from 5 documents?
If it's the former then you could use a scripted field as the field for the average metric agg. If the values are in different documents then I'm afraid you won't be able to accomplish this without pipeline aggregations.
timelion is a great tool for this type of metric calculation, and will absolutely help you do just that.