### 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
assertThat(histoBreakdown.get(BUILD_AGGREGATION), greaterThan(0L));
            assertThat(histoBreakdown.get(REDUCE), equalTo(0L));
            assertMap(
                histoAggResult.getDebugInfo(),
                matchesMap().entry(TOTAL_BUCKETS, greaterThan(0L)).entry(BUILT_BUCKETS, greaterThan(0))
            );
            ProfileResult termsAggResult = histoAggResult.getProfiledChildren().get(0);
            assertThat(termsAggResult, notNullValue());
            assertThat(termsAggResult.getQueryName(), equalTo(GlobalOrdinalsStringTermsAggregator.class.getSimpleName()));
            assertThat(termsAggResult.getLuceneDescription(), equalTo("terms"));
            assertThat(termsAggResult.getTime(), greaterThan(0L));
            Map<String, Long> termsBreakdown = termsAggResult.getTimeBreakdown();
            assertThat(termsBreakdown, notNullValue());
            assertThat(termsBreakdown.keySet(), equalTo(BREAKDOWN_KEYS));
            assertThat(termsBreakdown.get(INITIALIZE), greaterThan(0L));
            assertThat(termsBreakdown.get(COLLECT), greaterThan(0L));
            assertThat(termsBreakdown.get(BUILD_AGGREGATION), greaterThan(0L));
            assertThat(termsBreakdown.get(REDUCE), equalTo(0L));
            assertRemapTermsDebugInfo(termsAggResult);
            assertThat(termsAggResult.getProfiledChildren().size(), equalTo(1));
            ProfileResult avgAggResult = termsAggResult.getProfiledChildren().get(0);
            assertThat(avgAggResult, notNullValue());
            assertThat(avgAggResult.getQueryName(), equalTo("AvgAggregator"));
            assertThat(avgAggResult.getLuceneDescription(), equalTo("avg"));
            assertThat(avgAggResult.getTime(), greaterThan(0L));
            Map<String, Long> avgBreakdown = avgAggResult.getTimeBreakdown();
            assertThat(avgBreakdown, notNullValue());
            assertThat(avgBreakdown.keySet(), equalTo(BREAKDOWN_KEYS));
            assertThat(avgBreakdown.get(INITIALIZE), greaterThan(0L));
            assertThat(avgBreakdown.get(COLLECT), greaterThan(0L));