Unneccessary search source creation in vis courier request handler

[stacey-gammon]

I'm trying to track down the flow of search source creation and inheritance and ran into the code here:

            for (const agg of vis.getAggConfig()) {
              const nestedSearchSource = new SearchSource().inherits(searchSource);
              resp = await agg.type.postFlightRequest(resp, vis.aggs, agg, nestedSearchSource);
            }

It appears this creates a new search source for every aggregation, even though each search source looks exactly the same. I did a quick test of replacing nestedSearchSource with just searchSource and everything seems to be working fine, but I wasn't overly thorough (part of the reason I suggested a test be added for the issue #16643 fixed).

cc @elastic/kibana-visualizations

related to #16641

[ppisljar]

new searchsource is created as postFlightRequest will modify the searchsource to issue a new request. We still need the original one as spy panel will inspect it to show request/response information.

Possible perf. improvement would be to send the original searchsource in and rely on postFlightRequest implementation to create a new one in case it will need it. However that could lead to problems if postFlightRequest does not implement this correctly (modifies the passed in searchsource).

This will no longer be required once @timroes finishes with migration from spy panels to inspector

[timroes]

I mark this as blocked, and we will refactor this as soon as #16185 is ready.

[cjcenizal]

@timroes Are you planning on tackling this now that the inspector has been merged?

[timroes]

Will look at this next week. I think we can change it to have the postFlightRequestHandler actually create the child search source when it needs it. I'll discuss this with Peter on Monday.