moved _doc_count field from rollup._doc_count to root of document (#439) (#442)

Signed-off-by: Petar Dzepina <petar.dzepina@gmail.com>
(cherry picked from commit 1f581dd)

Co-authored-by: Petar Dzepina <petar.dzepina@gmail.com>

Author: opensearch-trigger-bot[bot]

src/main/kotlin/org/opensearch/indexmanagement/rollup/model/Rollup.kt

@@ -241,7 +241,10 @@ data class Rollup(
         const val MINIMUM_PAGE_SIZE = 1
         const val MAXIMUM_PAGE_SIZE = 10_000
         const val ROLLUP_DOC_ID_FIELD = "$ROLLUP_TYPE.$_ID"
-        const val ROLLUP_DOC_COUNT_FIELD = "$ROLLUP_TYPE._doc_count"
+        /*
+        *  _doc_count has to be in root of document so that core's aggregator would pick it up and use it
+        * */
+        const val ROLLUP_DOC_COUNT_FIELD = "_doc_count"
         const val ROLLUP_DOC_SCHEMA_VERSION_FIELD = "$ROLLUP_TYPE._$SCHEMA_VERSION_FIELD"
         const val USER_FIELD = "user"