[Draft] [Star Tree] [Search] Resolving Date histogram with metric aggregation using star-tree

Description

This is the draft set of changes which I am utilizing to discuss solution approach here.

The primary challenge to resolve a date histogram with metric aggregation was to figure out how sub-aggregators will get resolve. When resolving a query by star-tree, we lose lthe need of ucene documents and don't utilize collect() calls which are used internally to delegate the collection of sub-aggregations to sub-collectors.

To mitigate this challenge, I have introduced a wrapper class - StarTreeBucketCollector to basically introduce a collectStarEntry(int starTreeEntry, long bucket) method. This method is then overridden in metric aggregator methods and invoked from parent aggregators (here DateHistogramAggregator).

The benefit of this strategy is that this is easily extensible by other bucket aggregators where metric aggregations will be nested. Also, other bucket related utilities are re-useable as it is, it saves the effort of having a separate set of utilities for star tree buckets as the old buckets are utilized here.

Want to take early feedback on this approach.

Note: Things are hard-coded for one example query shape right now

{
    "size": 0,
    "aggs": {
        "by_hour": {
            "date_histogram": {
                "field": "@timestamp",
                "calendar_interval": "month"
            }, "aggs": {
                "sum_status": {
                    "sum": {
                        "field": "status"
                    }
                }
            }
        }
    }
}

{
    "took": 15785,
    "timed_out": false,
    "_shards": {
        "total": 1,
        "successful": 1,
        "skipped": 0,
        "failed": 0
    },
    "hits": {
        "total": {
            "value": 1000,
            "relation": "eq"
        },
        "max_score": null,
        "hits": []
    },
    "aggregations": {
        "by_hour": {
            "buckets": [
                {
                    "key_as_string": "2024-01-01T00:00:00.000Z",
                    "key": 1704067200000,
                    "doc_count": 208,
                    "sum_status": {
                        "value": 43527.0
                    }
                },
                {
                    "key_as_string": "2024-02-01T00:00:00.000Z",
                    "key": 1706745600000,
                    "doc_count": 783,
                    "sum_status": {
                        "value": 164741.0
                    }
                },
                {
                    "key_as_string": "2024-03-01T00:00:00.000Z",
                    "key": 1709251200000,
                    "doc_count": 9,
                    "sum_status": {
                        "value": 1894.0
                    }
                }
            ]
        }
    }
}

Check List

[ ] Functionality includes testing.
[ ] API changes companion pull request created, if applicable.
[ ] Public documentation issue/PR created, if applicable.

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opensearch-project / OpenSearch

[Draft] [Star Tree] [Search] Resolving Date histogram with metric aggregation using star-tree #16674

Description

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Check List