### 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
DateFieldMapper.DateFieldType ft,
        List<String> data,
        DateHistogramAggregationBuilder builder,
        boolean usesFromRange
    ) throws IOException {
        aggregationImplementationChoiceTestCase(ft, data, data, builder, usesFromRange);
    }
    private void aggregationImplementationChoiceTestCase(
        DateFieldMapper.DateFieldType ft,
        List<String> data,
        List<String> resultingBucketKeys,
        DateHistogramAggregationBuilder builder,
        boolean usesFromRange
    ) throws IOException {
        try (Directory directory = newDirectory(); RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory)) {
            for (String d : data) {
                long instant = asLong(d, ft);
                indexWriter.addDocument(
                    List.of(new SortedNumericDocValuesField(AGGREGABLE_DATE, instant), new LongPoint(AGGREGABLE_DATE, instant))
                );
            }
            try (IndexReader reader = indexWriter.getReader()) {
                AggregationContext context = createAggregationContext(new IndexSearcher(reader), new MatchAllDocsQuery(), ft);
                Aggregator agg = createAggregator(builder, context);
                Matcher<Aggregator> matcher = instanceOf(DateHistogramAggregator.FromDateRange.class);
                if (usesFromRange == false) {
                    matcher = not(matcher);
                }
                assertThat(agg, matcher);
                agg.preCollection();