Fix IndexStatsIT#testThrottleStats

`Engine.PauseLock#throttle` can be called when the lock is being throttled,
so we can't guarantee that all permits are available before throttling.

Resolve elastic#126359
See elastic#127173

Author: arteam

muted-tests.yml

@@ -450,9 +450,6 @@ tests:
 - class: org.elasticsearch.cluster.routing.allocation.decider.DiskThresholdDeciderIT
   method: testRestoreSnapshotAllocationDoesNotExceedWatermarkWithMultipleRestores
   issue: https://github.com/elastic/elasticsearch/issues/127787
-- class: org.elasticsearch.indices.stats.IndexStatsIT
-  method: testThrottleStats
-  issue: https://github.com/elastic/elasticsearch/issues/126359
 - class: org.elasticsearch.search.vectors.IVFKnnFloatVectorQueryTests
   method: testRandomWithFilter
   issue: https://github.com/elastic/elasticsearch/issues/127963

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -612,7 +612,6 @@ public Condition newCondition() {
         }
 
         public void throttle() {
-            assert semaphore.availablePermits() == Integer.MAX_VALUE;
             semaphore.acquireUninterruptibly(Integer.MAX_VALUE - allowThreads);
         }