Add fast path for single value in VALUES aggregator

[dnhatn]

This change introduces a fast path for the VALUES aggregator in the single-value case. For the first value seen in each group, we add it the new big array without touching the hash. For subsequent values, if they are the same as the current value, we skip them; if they differ, we trigger the slow path and add them to the hash. This optimization speeds up VALUES when the number of groups is large and most groups have only one value.

Before:

Benchmark                      (dataType)  (groups)  Mode  Cnt      Score       Error  Units
ValuesAggregatorBenchmark.run    BytesRef         1  avgt    3    177.756 ±     2.111  ms/op
ValuesAggregatorBenchmark.run    BytesRef      1000  avgt    3    126.174 ±     0.431  ms/op
ValuesAggregatorBenchmark.run    BytesRef   1000000  avgt    3  66920.144 ± 53588.490  ms/op

After:

Benchmark                      (dataType)  (groups)  Mode  Cnt      Score      Error  Units
ValuesAggregatorBenchmark.run    BytesRef         1  avgt    3    180.269 ±    4.019  ms/op
ValuesAggregatorBenchmark.run    BytesRef      1000  avgt    3    107.051 ±    3.149  ms/op
ValuesAggregatorBenchmark.run    BytesRef   1000000  avgt    3  26277.863 ± 7214.319  ms/op

[elasticsearchmachine]

Hi @dnhatn, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[nik9000]

I like the idea of collecting the first values into only the firstValues array. So we only go to the hash if there's more than one value. I'll have another look in the morning. I think it's correct, but I'll need another scan to make sure.

[nik9000]

Scratch can go below the quick bail outs.

[nik9000]

This isn't the counts any more - it's the counts in values, which is one less than the actual number of values. That's tricky. I'm not entirely sure what's correct here.