Fix AdaptiveSelectionStats serialization bug (#28718)

The AdaptiveSelectionStats object serializes the clientOutgoingConnections map that's concurrently updated in SearchTransportService. Serializing the map consists of first writing the size of the map and then serializing the entries. If the number of entries changes while the map is being serialized, the size and number of entries go out of sync. The deserialization routine expects those to be in sync though.

Closes #28713

server/src/main/java/org/elasticsearch/action/search/SearchTransportService.java

@@ -95,10 +95,6 @@ public SearchTransportService(Settings settings, TransportService transportServi
         this.responseWrapper = responseWrapper;
     }
 
-    public Map<String, Long> getClientConnections() {
-        return Collections.unmodifiableMap(clientConnections);
-    }
-
     public void sendFreeContext(Transport.Connection connection, final long contextId, OriginalIndices originalIndices) {
         transportService.sendRequest(connection, FREE_CONTEXT_ACTION_NAME, new SearchFreeContextRequest(originalIndices, contextId),
             TransportRequestOptions.EMPTY, new ActionListenerResponseHandler<>(new ActionListener<SearchFreeContextResponse>() {

server/src/main/java/org/elasticsearch/node/NodeService.java

@@ -121,7 +121,7 @@ public NodeStats stats(CommonStatsFlags indices, boolean os, boolean process, bo
                 script ? scriptService.stats() : null,
                 discoveryStats ? discovery.stats() : null,
                 ingest ? ingestService.getPipelineExecutionService().stats() : null,
-                adaptiveSelection ? responseCollectorService.getAdaptiveStats(searchTransportService.getClientConnections()) : null
+                adaptiveSelection ? responseCollectorService.getAdaptiveStats(searchTransportService.getPendingSearchRequests()) : null
         );
     }