Add setting to use document replication for system indices. (#802)

Signed-off-by: Rishikesh1159 <[email protected]>
(cherry picked from commit 55b6968)

src/main/kotlin/org/opensearch/replication/metadata/store/ReplicationMetadataStore.kt

@@ -40,6 +40,7 @@ import org.opensearch.common.xcontent.LoggingDeprecationHandler
 import org.opensearch.core.xcontent.NamedXContentRegistry
 import org.opensearch.core.xcontent.ToXContent
 import org.opensearch.core.xcontent.XContentParser
+import org.opensearch.indices.replication.common.ReplicationType
 import org.opensearch.replication.util.suspendExecuteWithRetries
 
 class ReplicationMetadataStore constructor(val client: Client, val clusterService: ClusterService,
@@ -265,6 +266,7 @@ class ReplicationMetadataStore constructor(val client: Client, val clusterServic
                 .put(IndexMetadata.INDEX_AUTO_EXPAND_REPLICAS_SETTING.key, "0-1")
                 .put(IndexMetadata.INDEX_PRIORITY_SETTING.key, Int.MAX_VALUE)
                 .put(IndexMetadata.INDEX_HIDDEN_SETTING.key, true)
+                .put(IndexMetadata.INDEX_REPLICATION_TYPE_SETTING.key, ReplicationType.DOCUMENT) // System Indices should use Document Replication strategy
                 .build()
     }