I'm not sure there's a need for it. When a query runs under auto_explain, the output values will be sent to the client, so those cycles should be accounted for anyway, no?
Yes, great point, the total duration reported by auto_explain includes it. Explicit serialization stats might still be helpful for folks when it is the bottleneck, but less useful for sure (especially if nothing else causes big discrepancies between the duration reported by auto_explain and the "actual total time" of the root node).
(Perhaps the auto_explain documentation should mention this?)
I'd value this. I notice the folks working on the other new explain parameter (memory) opted to add a comment to the auto_explain source code to say it wasn't supported.