Pass empty QueryCollectorContext in case of hybrid query to improve latencies by 20%

[martin-gaievski]

Description

In this change we're improving hybrid query latencies by avoiding redundant document score collection.

As per flamegraphs posted in #729 from 40 to 80% of CPU time of hybrid query processing is taken by TopDocsCollector in core. Those scores are not needed and should be removed from the execution flow in case this is hybrid query.

At the hight level the idea is to set empty doc collector instead of top docs collector in case incoming query is of type hybrid.

Ability to pass empty doc collector has been added to core in earlier PR. In this PR we're using this feature in neural plugin by passing new empty query collector context to query phase searcher. This is done by overriding searchWith method of both default and concurrent query phase searchers. The only collector and collector manager that will be executed for hybrid query are custom plugin implementations: HybridTopScoreDocCollector and HybridCollectorManager.

Below are metrics for hybrid query latency that I've collected after this change. There are based on 2.x (2.14) version and noaa OSB workload, all times are in ms:

One sub-query that selects 11M documents

Bool: p50 80.0931 | p90 80.4647
Hybrid: p50 182.314 | p90 204.263

One sub-query that selects 1.6K documents

Bool: p50 73.1325 | p90 73.7713
Hybrid: p50 72.9788 | p90 73.211

Three sub-query that select 15M documents

Bool: p50 85.8744 | p90 90.3619
Hybrid: p50 274.728 | p90 293.725

following is the baseline results, measurements are done before this change

One sub-query that selects 11M documents

Bool: p50 97.9306 | p90 116.299
Hybrid: p50 228.696 | p90 249.665

One sub-query that selects 1.6K documents

Bool: p50 87.3152 | p90 89.3061
Hybrid: p50 89.9654 | p90 92.349

Three sub-query that select 15M documents

Bool: p50 97.9891 | p90 114.396
Hybrid: p50 353.631 | p90 377.527

Following flamegraph (taken after this change) shows that there is no call to TopDocsCollector. More deep dive is needed to find next focus area for optimizations:

Issues Resolved

#729
#704

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has javadoc added
• Commits are signed as per the DCO using --signoff

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[martin-gaievski]

BWC rolling upgrade tests will fail for 2.14 while the release is ongoing and core is already switched to 2.15 for 2.x branch, restart BWCs are fine

[VijayanB]

LGTM.