### 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
);
        List<IndexRequestBuilder> builders = new ArrayList<>();
        String[] randomStrings = new String[randomIntBetween(2, 10)];
        for (int i = 0; i < randomStrings.length; i++) {
            randomStrings[i] = randomAlphaOfLength(10);
        }
        for (int i = 0; i < 5; i++) {
            builders.add(
                client().prepareIndex("idx")
                    .setSource(
                        jsonBuilder().startObject()
                            .field(STRING_FIELD, randomFrom(randomStrings))
                            .field(NUMBER_FIELD, randomIntBetween(0, 9))
                            .field(TAG_FIELD, randomBoolean() ? "more" : "less")
                            .endObject()
                    )
            );
        }
        indexRandom(true, false, builders);
        createIndex("idx_unmapped");
    }
    public void testSimpleProfile() {
        SearchResponse response = client().prepareSearch("idx")
            .setProfile(true)
            .addAggregation(histogram("histo").field(NUMBER_FIELD).interval(1L))
            .get();
        assertSearchResponse(response);
        Map<String, SearchProfileShardResult> profileResults = response.getProfileResults();
        assertThat(profileResults, notNullValue());
        assertThat(profileResults.size(), equalTo(getNumShards("idx").numPrimaries));