Do not check self references in metric agg per document

[jonaslan]

#31044 introduced ensureNoSelfReference on the _agg object in scripted metric aggregations.
It would be nice if there was a switch to disable the it. Upgrading to 6.4 caused something around 20x performance degradation for us. And downgrading was no fun.

[elasticmachine]

Pinging @elastic/es-search-aggs

[jpountz]

I agree #31044 was not good, it should not check for self references in the aggregation state after every collected document, only once all documents are collected.

[rjernst]

We discussed in FixItThursday and believe Adrien's idea should work. @polyfractal suggested doing this check in a post collection hook. I'm marking this as adoptme and updating the title accordingly.

[cbismuth]

I'm having a look at this one.

[cbismuth]

I've opened PR #34001, could you please have a look on it? Thank you.