Save a call to prepareRounding if roundingIdx is unchanged.

Signed-off-by: Finn Carroll <carrofin@amazon.com>

server/src/main/java/org/opensearch/search/aggregations/bucket/histogram/AutoDateHistogramAggregator.java

@@ -203,7 +203,10 @@ protected Rounding getRounding(final long low, final long high) {
 
                 // Ensure preparedRounding never shrinks
                 roundingIdx = Math.max(prevRoundingIdx, roundingIdx);
-                preparedRounding = prepareRounding(roundingIdx);
+                if (roundingIdx != prevRoundingIdx) {
+                    preparedRounding = prepareRounding(roundingIdx);
+                }
+
                 return roundingInfos[roundingIdx].rounding;
             }