### 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
randomBoolean(),
            randomBoolean(),
            true,
            false,
            null,
            Collections.emptyMap(),
            null,
            false,
            null
        );
        RangeAggregationBuilder aggregationBuilder = new RangeAggregationBuilder("test_range_agg");
        aggregationBuilder.field(NUMBER_FIELD_NAME);
        aggregationBuilder.addRange(0d, 5d);
        aggregationBuilder.addRange(10d, 20d);
        testCase(aggregationBuilder, query, buildIndex, verify, fieldType);
    }
    private void simpleTestCase(
        RangeAggregationBuilder aggregationBuilder,
        Query query,
        Consumer<InternalRange<? extends InternalRange.Bucket, ? extends InternalRange<?, ?>>> verify
    ) throws IOException {
        MappedFieldType fieldType = new NumberFieldMapper.NumberFieldType(NUMBER_FIELD_NAME, NumberFieldMapper.NumberType.INTEGER);
        testCase(aggregationBuilder, new MatchAllDocsQuery(), iw -> {
            iw.addDocument(singleton(new SortedNumericDocValuesField(NUMBER_FIELD_NAME, 7)));
            iw.addDocument(singleton(new SortedNumericDocValuesField(NUMBER_FIELD_NAME, 2)));
            iw.addDocument(singleton(new SortedNumericDocValuesField(NUMBER_FIELD_NAME, 3)));
        }, verify, fieldType);
    }
}