Remove QueryCollectorContext abstraction

[javanna]

The QueryCollectorContext abstraction was introduced by #24864 based on the requirement that the top docs collector creation needed to be delayed until after all the other collectors had been created. At the same time, collectors get wrapped depending on the search features enabled by the request, but the top score / total hit count collector is the root collector where the wrapping starts, which is why its corresponding context gets added at position 0 in the list of collector contexts.

Requirements have changed since #27666 , which means that we can go back to a simpler way of creating collectors and wrapping them. We no longer need a QueryCollectorContext abstraction, and we can instead create collectors straight-away, and wrap them as needed. This is much easier to follow compared to the very generic create(Collector) method that the context exposes.

TopDocsCollectorContext adds some value in that it incorporates all the logic around creating the top docs collector, yet it can be further simplified as well by making the postProcess method more specific.

[iverase]

I wonder if we can move this complexity in the wrapWithProfilerCollectorIfNeeded method. The ide awould be to use varargs in the method like:

 private static Collector wrapWithProfilerCollectorIfNeeded(
        Profilers profilers,
        Collector collector,
        String profilerName,
        Collector... children
    ) {
        if (profilers == null) {
            return collector;
        }
        final List<InternalProfileCollector> profileCollectors;
        if (children == null) {
            profileCollectors = List.of();
        } else {
            profileCollectors = new ArrayList<>(children.length);
            Arrays.stream(children).forEach(c -> profileCollectors.add((InternalProfileCollector) c));
        }
        return new InternalProfileCollector(
            collector,
            profilerName,
            profileCollectors
        );
    }

So here we can just call the method like:

// plug in additional collectors, like aggregations
                // in this case we pass both collectors as children profile collectors
                collector = wrapWithProfilerCollectorIfNeeded(
                    searchContext.getProfilers(),
                    MultiCollector.wrap(collector, searchContext.getAggsCollector()),
                    REASON_SEARCH_MULTI,
                    collector,
                    searchContext.getAggsCollector());

[javanna]

I tried that, it looked a bit awkward. I will try again.

[javanna]

I think this will work, #95427 will also help in that direction so that InternalProfileCollector no longer requires a list.

[javanna]

The casting needed was a bit on the way and did not combine well with the varargs, but I found a way to reuse the wrap method for aggs too. It's not perfect but it's better than before I think.

[iverase]

I like the direction of this PR, it makes the code much easier to follow.

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[iverase]

LGTM