Speed up reducing auto_date_histo with a time zone (#57933)

When reducing `auto_date_histogram` we were using `Rounding#round`
which is quite a bit more expensive than
```
Rounding.Prepared prepared = rounding.prepare(min, max);
long result = prepared.round(date);
```
when rounding to a non-fixed time zone like `America/New_York`. This
stops using the former and starts using the latter.

Relates to #56124

server/src/main/java/org/elasticsearch/search/aggregations/bucket/histogram/InternalAutoDateHistogram.java

@@ -293,15 +293,17 @@ public final class InternalAutoDateHistogram extends
         // First we need to find the highest level rounding used across all the
         // shards
         int reduceRoundingIdx = 0;
+        long min = Long.MAX_VALUE;
+        long max = Long.MIN_VALUE;
         for (InternalAggregation aggregation : aggregations) {
-            int aggRoundingIdx = ((InternalAutoDateHistogram) aggregation).bucketInfo.roundingIdx;
-            if (aggRoundingIdx > reduceRoundingIdx) {
-                reduceRoundingIdx = aggRoundingIdx;
+            InternalAutoDateHistogram agg = ((InternalAutoDateHistogram) aggregation);
+            reduceRoundingIdx = Math.max(agg.bucketInfo.roundingIdx, reduceRoundingIdx);
+            if (false == agg.buckets.isEmpty()) {
+                min = Math.min(min, agg.buckets.get(0).key);
+                max = Math.max(max, agg.buckets.get(agg.buckets.size() - 1).key);
             }
         }
-        // This rounding will be used to reduce all the buckets
-        RoundingInfo reduceRoundingInfo = bucketInfo.roundingInfos[reduceRoundingIdx];
-        Rounding reduceRounding = reduceRoundingInfo.rounding;
+        Rounding.Prepared reduceRounding = prepare(reduceRoundingIdx, min, max);
 
         final PriorityQueue<IteratorAndCurrent> pq = new PriorityQueue<IteratorAndCurrent>(aggregations.size()) {
             @Override
@@ -351,21 +353,33 @@ public final class InternalAutoDateHistogram extends
             }
         }
 
-        return mergeBucketsIfNeeded(reducedBuckets, reduceRoundingIdx, reduceRoundingInfo, reduceContext);
+        return mergeBucketsIfNeeded(
+            new BucketReduceResult(reducedBuckets, reduceRoundingIdx, 1, reduceRounding, min, max),
+            reduceContext
+        );
     }
 
-    private BucketReduceResult mergeBucketsIfNeeded(List<Bucket> reducedBuckets, int reduceRoundingIdx, RoundingInfo reduceRoundingInfo,
-            ReduceContext reduceContext) {
-        while (reducedBuckets.size() > (targetBuckets * reduceRoundingInfo.getMaximumInnerInterval())
-                && reduceRoundingIdx < bucketInfo.roundingInfos.length - 1) {
-            reduceRoundingIdx++;
-            reduceRoundingInfo = bucketInfo.roundingInfos[reduceRoundingIdx];
-            reducedBuckets = mergeBuckets(reducedBuckets, reduceRoundingInfo.rounding, reduceContext);
-        }
-        return new BucketReduceResult(reducedBuckets, reduceRoundingInfo, reduceRoundingIdx, 1);
+    private BucketReduceResult mergeBucketsIfNeeded(BucketReduceResult firstPassResult, ReduceContext reduceContext) {
+        int idx = firstPassResult.roundingIdx;
+        RoundingInfo info = bucketInfo.roundingInfos[idx];
+        List<Bucket> buckets = firstPassResult.buckets;
+        Rounding.Prepared prepared = firstPassResult.preparedRounding;
+        while (buckets.size() > (targetBuckets * info.getMaximumInnerInterval())
+                && idx < bucketInfo.roundingInfos.length - 1) {
+            idx++;
+            info = bucketInfo.roundingInfos[idx];
+            prepared = prepare(idx, firstPassResult.min, firstPassResult.max);
+            buckets = mergeBuckets(buckets, prepared, reduceContext);
+        }
+        return new BucketReduceResult(buckets, idx, 1, prepared, firstPassResult.min, firstPassResult.max);
+    }
+
+    private Rounding.Prepared prepare(int idx, long min, long max) {
+        Rounding rounding = bucketInfo.roundingInfos[idx].rounding;
+        return min <= max ? rounding.prepare(min, max) : rounding.prepareForUnknown();
     }
 
-    private List<Bucket> mergeBuckets(List<Bucket> reducedBuckets, Rounding reduceRounding, ReduceContext reduceContext) {
+    private List<Bucket> mergeBuckets(List<Bucket> reducedBuckets, Rounding.Prepared reduceRounding, ReduceContext reduceContext) {
         List<Bucket> mergedBuckets = new ArrayList<>();
 
         List<Bucket> sameKeyedBuckets = new ArrayList<>();
@@ -405,35 +419,51 @@ public final class InternalAutoDateHistogram extends
     }
 
     private static class BucketReduceResult {
-        List<Bucket> buckets;
-        RoundingInfo roundingInfo;
-        int roundingIdx;
-        long innerInterval;
-
-        BucketReduceResult(List<Bucket> buckets, RoundingInfo roundingInfo, int roundingIdx, long innerInterval) {
+        final List<Bucket> buckets;
+        final int roundingIdx;
+        final long innerInterval;
+        final Rounding.Prepared preparedRounding;
+        final long min;
+        final long max;
+
+        BucketReduceResult(
+            List<Bucket> buckets,
+            int roundingIdx,
+            long innerInterval,
+            Rounding.Prepared preparedRounding,
+            long min,
+            long max
+        ) {
             this.buckets = buckets;
-            this.roundingInfo = roundingInfo;
             this.roundingIdx = roundingIdx;
             this.innerInterval = innerInterval;
+            this.preparedRounding = preparedRounding;
+            this.min = min;
+            this.max = max;
         }
     }
 
-    private BucketReduceResult addEmptyBuckets(BucketReduceResult currentResult, ReduceContext reduceContext) {
-        List<Bucket> list = currentResult.buckets;
+    private BucketReduceResult addEmptyBuckets(BucketReduceResult current, ReduceContext reduceContext) {
+        List<Bucket> list = current.buckets;
         if (list.isEmpty()) {
-            return currentResult;
-        }
-        int roundingIdx = getAppropriateRounding(list.get(0).key, list.get(list.size() - 1).key, currentResult.roundingIdx,
-                bucketInfo.roundingInfos, targetBuckets);
-        RoundingInfo roundingInfo = bucketInfo.roundingInfos[roundingIdx];
-        Rounding rounding = roundingInfo.rounding;
+            return current;
+        }
+        int roundingIdx = getAppropriateRounding(
+            list.get(0).key,
+            list.get(list.size() - 1).key,
+            current.roundingIdx,
+            bucketInfo.roundingInfos,
+            targetBuckets
+        );
+        Rounding.Prepared rounding = current.roundingIdx == roundingIdx
+            ? current.preparedRounding
+            : prepare(roundingIdx, current.min, current.max);
         // merge buckets using the new rounding
         list = mergeBuckets(list, rounding, reduceContext);
 
         Bucket lastBucket = null;
         ListIterator<Bucket> iter = list.listIterator();
-        InternalAggregations reducedEmptySubAggs = InternalAggregations.reduce(Collections.singletonList(bucketInfo.emptySubAggregations),
-                reduceContext);
+        InternalAggregations reducedEmptySubAggs = InternalAggregations.reduce(List.of(bucketInfo.emptySubAggregations), reduceContext);
 
         // Add the empty buckets within the data,
         // e.g. if the data series is [1,2,3,7] there're 3 empty buckets that will be created for 4,5,6
@@ -449,7 +479,7 @@ public final class InternalAutoDateHistogram extends
             }
             lastBucket = iter.next();
         }
-        return new BucketReduceResult(list, roundingInfo, roundingIdx, currentResult.innerInterval);
+        return new BucketReduceResult(list, roundingIdx, 1, rounding, current.min, current.max);
     }
 
     static int getAppropriateRounding(long minKey, long maxKey, int roundingIdx,
@@ -501,8 +531,7 @@ public final class InternalAutoDateHistogram extends
             reducedBucketsResult = addEmptyBuckets(reducedBucketsResult, reduceContext);
 
             // Adding empty buckets may have tipped us over the target so merge the buckets again if needed
-            reducedBucketsResult = mergeBucketsIfNeeded(reducedBucketsResult.buckets, reducedBucketsResult.roundingIdx,
-                    reducedBucketsResult.roundingInfo, reduceContext);
+            reducedBucketsResult = mergeBucketsIfNeeded(reducedBucketsResult, reduceContext);
 
             // Now finally see if we need to merge consecutive buckets together to make a coarser interval at the same rounding
             reducedBucketsResult = maybeMergeConsecutiveBuckets(reducedBucketsResult, reduceContext);
@@ -515,11 +544,9 @@ public final class InternalAutoDateHistogram extends
                 getMetadata(), reducedBucketsResult.innerInterval);
     }
 
-    private BucketReduceResult maybeMergeConsecutiveBuckets(BucketReduceResult reducedBucketsResult,
-                                                                   ReduceContext reduceContext) {
-        List<Bucket> buckets = reducedBucketsResult.buckets;
-        RoundingInfo roundingInfo = reducedBucketsResult.roundingInfo;
-        int roundingIdx = reducedBucketsResult.roundingIdx;
+    private BucketReduceResult maybeMergeConsecutiveBuckets(BucketReduceResult current, ReduceContext reduceContext) {
+        List<Bucket> buckets = current.buckets;
+        RoundingInfo roundingInfo = bucketInfo.roundingInfos[current.roundingIdx];
         if (buckets.size() > targetBuckets) {
             for (int interval : roundingInfo.innerIntervals) {
                 int resultingBuckets = buckets.size() / interval;
@@ -527,32 +554,38 @@ public final class InternalAutoDateHistogram extends
                     resultingBuckets++;
                 }
                 if (resultingBuckets <= targetBuckets) {
-                    return mergeConsecutiveBuckets(buckets, interval, roundingIdx, roundingInfo, reduceContext);
+                    return mergeConsecutiveBuckets(current, interval, reduceContext);
                 }
             }
         }
-        return reducedBucketsResult;
+        return current;
     }
 
-    private BucketReduceResult mergeConsecutiveBuckets(List<Bucket> reducedBuckets, int mergeInterval, int roundingIdx,
-            RoundingInfo roundingInfo, ReduceContext reduceContext) {
+    private BucketReduceResult mergeConsecutiveBuckets(BucketReduceResult current, int mergeInterval, ReduceContext reduceContext) {
         List<Bucket> mergedBuckets = new ArrayList<>();
         List<Bucket> sameKeyedBuckets = new ArrayList<>();
 
-        double key = roundingInfo.rounding.round(reducedBuckets.get(0).key);
-        for (int i = 0; i < reducedBuckets.size(); i++) {
-            Bucket bucket = reducedBuckets.get(i);
+        double key = current.preparedRounding.round(current.buckets.get(0).key);
+        for (int i = 0; i < current.buckets.size(); i++) {
+            Bucket bucket = current.buckets.get(i);
             if (i % mergeInterval == 0 && sameKeyedBuckets.isEmpty() == false) {
                 mergedBuckets.add(reduceBucket(sameKeyedBuckets, reduceContext));
                 sameKeyedBuckets.clear();
-                key = roundingInfo.rounding.round(bucket.key);
+                key = current.preparedRounding.round(bucket.key);
             }
             sameKeyedBuckets.add(new Bucket(Math.round(key), bucket.docCount, format, bucket.aggregations));
         }
         if (sameKeyedBuckets.isEmpty() == false) {
             mergedBuckets.add(reduceBucket(sameKeyedBuckets, reduceContext));
         }
-        return new BucketReduceResult(mergedBuckets, roundingInfo, roundingIdx, mergeInterval);
+        return new BucketReduceResult(
+            mergedBuckets,
+            current.roundingIdx,
+            mergeInterval,
+            current.preparedRounding,
+            current.min,
+            current.max
+        );
     }
 
     @Override