### 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/range/RangeAggregatorTests.java
},
            (InternalRange<?, ?> r, Class<? extends Aggregator> impl, Map<String, Map<String, Object>> debug) -> {
                assertThat(
                    r.getBuckets().stream().map(InternalRange.Bucket::getKeyAsString).collect(toList()),
                    equalTo(List.of("0.0-1.0", "1.0-2.0", "2.0-3.0"))
                );
                assertThat(r.getBuckets().stream().map(InternalRange.Bucket::getFrom).collect(toList()), equalTo(List.of(0.0, 1.0, 2.0)));
                assertThat(r.getBuckets().stream().map(InternalRange.Bucket::getTo).collect(toList()), equalTo(List.of(1.0, 2.0, 3.0)));
                assertThat(
                    r.getBuckets().stream().map(InternalRange.Bucket::getDocCount).collect(toList()),
                    equalTo(List.of(totalDocs, 0L, 0L))
                );
                assertThat(impl, equalTo(RangeAggregator.NoOverlap.class));
                assertMap(
                    debug,
                    matchesMap().entry("r", matchesMap().entry("ranges", 3).entry("average_docs_per_range", closeTo(6667, 1)))
                );
            },
            dummyFt,
            numberFt
        );
    }
    private void testCase(
        Query query,
        CheckedConsumer<RandomIndexWriter, IOException> buildIndex,
        Consumer<InternalRange<? extends InternalRange.Bucket, ? extends InternalRange<?, ?>>> verify
    ) throws IOException {
        MappedFieldType fieldType = new NumberFieldMapper.NumberFieldType(
            NUMBER_FIELD_NAME,
            NumberFieldMapper.NumberType.INTEGER,