### 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/main/java/org/elasticsearch/search/aggregations/bucket/range/RangeAggregator.java
);
    }
    @Override
    @SuppressWarnings("unchecked")
    public InternalAggregation buildEmptyAggregation() {
        InternalAggregations subAggs = buildEmptySubAggregations();
        List<org.elasticsearch.search.aggregations.bucket.range.Range.Bucket> buckets = new ArrayList<>(ranges.length);
        for (int i = 0; i < ranges.length; i++) {
            Range range = ranges[i];
            org.elasticsearch.search.aggregations.bucket.range.Range.Bucket bucket = rangeFactory.createBucket(
                range.key,
                range.originalFrom,
                range.originalTo,
                0,
                subAggs,
                keyed,
                format
            );
            buckets.add(bucket);
        }
        
        return rangeFactory.create(name, buckets, format, keyed, metadata());
    }
    @Override
    public void collectDebugInfo(BiConsumer<String, Object> add) {
        super.collectDebugInfo(add);
        add.accept("ranges", ranges.length);
        add.accept("average_docs_per_range", averageDocsPerRange);
    }
    public static class Unmapped<R extends RangeAggregator.Range> extends NonCollectingAggregator {