### 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
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))));
        }, new NumberFieldMapper.NumberFieldType(NUMBER_FIELD_NAME, NumberFieldMapper.NumberType.INTEGER));
    }
    /**
     * If the field we're getting the range of is a runtime field it'd be super
     * slow to run a bunch of range queries on it so we disable the optimization.
     */
    public void testRuntimeFieldRangesNotOptimized() throws IOException {
        long totalDocs = (long) RangeAggregator.DOCS_PER_RANGE_TO_USE_FILTERS * 4;
        LongFieldScript.Factory scriptFactory = (fieldName, params, l) -> ctx -> new LongFieldScript(fieldName, Map.of(), l, ctx) {
            @Override
            public void execute() {
                emit((long) getDoc().get(NUMBER_FIELD_NAME).get(0));
            }
        };
        MappedFieldType dummyFt = new LongScriptFieldType("dummy", scriptFactory, new Script("test"), Map.of());
        MappedFieldType numberFt = new NumberFieldMapper.NumberFieldType(NUMBER_FIELD_NAME, NumberFieldMapper.NumberType.INTEGER);
        debugTestCase(
            new RangeAggregationBuilder("r").field("dummy").addRange(0, 1).addRange(1, 2).addRange(2, 3),
            new MatchAllDocsQuery(),
            iw -> {
                for (int d = 0; d < totalDocs; d++) {
                    iw.addDocument(List.of(new IntPoint(NUMBER_FIELD_NAME, 0), new SortedNumericDocValuesField(NUMBER_FIELD_NAME, 0)));
                }