Base search queue length on pool size (elastic#116578)

Base the queue length on the pool size. The number of tasks enqueued
on search is proportional to its size in many cases so linearly
increaasing the length of the queue makes it so a larger pool does not
lead to an increasing chance of rejection under constant search load.

Author: original-brownbear

server/src/main/java/org/elasticsearch/threadpool/DefaultBuiltInExecutorBuilders.java

@@ -68,7 +68,7 @@ public Map<String, ExecutorBuilder> getBuilders(Settings settings, int allocated
                 settings,
                 ThreadPool.Names.SEARCH,
                 searchOrGetThreadPoolSize,
-                1000,
+                searchOrGetThreadPoolSize * 1000,
                 new EsExecutors.TaskTrackingConfig(true, searchAutoscalingEWMA)
             )
         );