### Description

Our most common aggregations should be optimized for single-valued fields. In general, our aggregations look like this:

```java
    @Override
    public LeafBucketCollector getLeafCollector(LeafReaderContext ctx, LeafBucketCollector sub) throws IOException {
        if (valuesSource == null) {
            return LeafBucketCollector.NO_OP_COLLECTOR;
        }
        SortedNumericDocValues values = valuesSource.longValues(ctx);
        return new LeafBucketCollectorBase(sub, values) {
            @Override
            public void collect(int doc, long owningBucketOrd) throws IOException {
                if (values.advanceExact(doc)) {
                    int valuesCount = values.docValueCount();
                    for (int i = 0; i < valuesCount; ++i) {
                        long value = values.nextValue();
                        // do something with value
                    }
                }
            }
        };
    }
```

The idea consists of optimizing for single-valued fields by unwrapping the single-valued doc-value iterator:

```java
    @Override
    public LeafBucketCollector getLeafCollector(LeafReaderContext ctx, LeafBucketCollector sub) throws IOException {
        if (valuesSource == null) {
            return LeafBucketCollector.NO_OP_COLLECTOR;
        }
        SortedNumericDocValues values = valuesSource.longValues(ctx);
        NumericDocValues singleValues = DocValues.unwrapSingleton(values); // or FieldData.unwrapSingleton for doubles
        if (singleValues != null) {
            return new LeafBucketCollectorBase(sub, values) {
                @Override
                public void collect(int doc, long owningBucketOrd) throws IOException {
                    if (singleValues.advanceExact(doc)) {
                        long value = singleValues.longValue();
                        // do something with value
                    }
                }
            };
        } else {
            return new LeafBucketCollectorBase(sub, values) {
                @Override
                public void collect(int doc, long owningBucketOrd) throws IOException {
                    if (values.advanceExact(doc)) {
                        int valuesCount = values.docValueCount();
                        for (int i = 0; i < valuesCount; ++i) {
                            long value = values.nextValue();
                            // do something with value
                        }
                    }
                }
            };
        }
    }
```

The `terms` aggregation already does this, and e.g. `min` and `max` aggregations use similar tricks that also help optimize the single-valued case. But `date_histogram`, which is one of our most common aggregations, doesn't. Let's specialize the single-value case for date histograms too?
server/src/test/java/org/elasticsearch/search/aggregations/bucket/histogram/DateHistogramAggregatorTests.java
)
                                    )
                            )
                    )
                );
            },
            ft,
            fnft
        );
    }
    /**
     * If there is a doc count field and more than one bucket it is
     * not faster to use filter-by-filter collection mode so we don't
     * use it.
     */
    public void testTwoBucketsWithDocCountUsesTraditionalCollection() throws IOException {
        AggregationBuilder builder = new DateHistogramAggregationBuilder("d").field("f").calendarInterval(DateHistogramInterval.DAY);
        CheckedConsumer<RandomIndexWriter, IOException> buildIndex = iw -> {
            long[] dates = new long[] {
                DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.parseMillis("2020-01-01T00:00:01"),
                DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER.parseMillis("2020-01-02T00:00:01") };
            for (int i = 0; i < RangeAggregator.DOCS_PER_RANGE_TO_USE_FILTERS * 2 + 10; i++) {
                long date = dates[i % 2];
                iw.addDocument(
                    List.of(
                        new LongPoint("f", date),
                        new NumericDocValuesField("f", date),
                        new CustomTermFreqField(DocCountFieldMapper.NAME, DocCountFieldMapper.NAME, 2)
                    )
                );