Increase the minimum size of the management pool to 2 (#90193)

Make sure that individual `wait_for_completion=true` requests don't block concurrent
stats and cat requests.

See #89564

docs/changelog/90193.yaml

@@ -0,0 +1,5 @@
+pr: 90193
+summary: Increase the minimum size of the management pool to 2
+area: Distributed
+type: enhancement
+issues: []

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

@@ -198,7 +198,7 @@ public class ThreadPool implements ReportingService<ThreadPoolInfo>, Scheduler {
         builders.put(Names.SEARCH_THROTTLED, new FixedExecutorBuilder(settings, Names.SEARCH_THROTTLED, 1, 100, true));
         builders.put(
             Names.MANAGEMENT,
-            new ScalingExecutorBuilder(Names.MANAGEMENT, 1, boundedBy(allocatedProcessors, 1, 5), TimeValue.timeValueMinutes(5), false)
+            new ScalingExecutorBuilder(Names.MANAGEMENT, 2, boundedBy(allocatedProcessors, 2, 5), TimeValue.timeValueMinutes(5), false)
         );
         builders.put(Names.FLUSH, new ScalingExecutorBuilder(Names.FLUSH, 1, halfProcMaxAt5, TimeValue.timeValueMinutes(5), false));
         builders.put(Names.REFRESH, new ScalingExecutorBuilder(Names.REFRESH, 1, halfProcMaxAt10, TimeValue.timeValueMinutes(5), false));

server/src/test/java/org/elasticsearch/threadpool/ScalingThreadPoolTests.java

@@ -106,7 +106,7 @@ public class ScalingThreadPoolTests extends ESThreadPoolTestCase {
     private int expectedSize(final String threadPoolName, final int numberOfProcessors) {
         final Map<String, Function<Integer, Integer>> sizes = new HashMap<>();
         sizes.put(ThreadPool.Names.GENERIC, n -> ThreadPool.boundedBy(4 * n, 128, 512));
-        sizes.put(ThreadPool.Names.MANAGEMENT, n -> ThreadPool.boundedBy(n, 1, 5));
+        sizes.put(ThreadPool.Names.MANAGEMENT, n -> ThreadPool.boundedBy(n, 2, 5));
         sizes.put(ThreadPool.Names.FLUSH, ThreadPool::halfAllocatedProcessorsMaxFive);
         sizes.put(ThreadPool.Names.REFRESH, ThreadPool::halfAllocatedProcessorsMaxTen);
         sizes.put(ThreadPool.Names.WARMER, ThreadPool::halfAllocatedProcessorsMaxFive);