Lower down the log level to DEBUG

This logging message is relevant to experimental feature that is not
yet turned on by default. When this code runs in production then this
logging message is confisung OPS because there is nothing they can do
to address this WARN level message.

Closes: #18137

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>

Author: lukas-vlcek

CHANGELOG.md

@@ -16,6 +16,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Changed
 
+- Lower down the log level to DEBUG ([#18173](https://github.com/opensearch-project/OpenSearch/pull/18173))
+
 ### Dependencies
 - Bump `com.google.code.gson:gson` from 2.12.1 to 2.13.0 ([#17923](https://github.com/opensearch-project/OpenSearch/pull/17923))
 - Bump `com.github.spotbugs:spotbugs-annotations` from 4.9.0 to 4.9.3 ([#17922](https://github.com/opensearch-project/OpenSearch/pull/17922))

server/src/main/java/org/opensearch/cluster/routing/allocation/AllocationService.java

@@ -625,7 +625,7 @@ private void allocateExistingUnassignedShards(RoutingAllocation allocation) {
             allocateAllUnassignedShards(allocation);
             return;
         }
-        logger.warn("Falling back to single shard assignment since batch mode disable or multiple custom allocators set");
+        logger.debug("Falling back to single shard assignment since batch mode disable or multiple custom allocators set");
 
         final RoutingNodes.UnassignedShards.UnassignedIterator primaryIterator = allocation.routingNodes().unassigned().iterator();
         while (primaryIterator.hasNext()) {