### 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/internalClusterTest/java/org/elasticsearch/search/profile/aggregation/AggregationProfilerIT.java
.entry(BUILD_LEAF_COLLECTOR, greaterThan(0L))
                        .entry(BUILD_LEAF_COLLECTOR + "_count", greaterThan(0L))
                        .entry(COLLECT, greaterThan(0L))
                        .entry(COLLECT + "_count", greaterThan(0L))
                        .entry(POST_COLLECTION, greaterThan(0L))
                        .entry(POST_COLLECTION + "_count", 1L)
                        .entry(BUILD_AGGREGATION, greaterThan(0L))
                        .entry(BUILD_AGGREGATION + "_count", greaterThan(0L))
                        .entry(REDUCE, 0L)
                        .entry(REDUCE + "_count", 0L)
                );
                Map<String, Object> debug = histoAggResult.getDebugInfo();
                assertMap(
                    debug,
                    matchesMap().entry("delegate", "RangeAggregator.NoOverlap")
                        .entry("built_buckets", 1)
                        .entry("delegate_debug", matchesMap().entry("ranges", 1).entry("average_docs_per_range", 10000.0))
                );
            }
        } finally {
            assertAcked(
                client().admin()
                    .cluster()
                    .prepareUpdateSettings()
                    .setPersistentSettings(Settings.builder().putNull(SearchService.ENABLE_REWRITE_AGGS_TO_FILTER_BY_FILTER.getKey()))
            );
        }
    }
}