EXT-1546 Fix monitoring race between shutdown and pages registration (#25921)

(cherry picked from commit c26aa1a)

Author: UgnineSirdis

ydb/core/mon/mon.cpp

@@ -1380,6 +1380,10 @@ TMon::TMon(TConfig config)
 {
 }
 
+TMon::~TMon() {
+    IndexMonPage->ClearPages(); // it's required to avoid loop-reference
+}
+
 std::future<void> TMon::Start(TActorSystem* actorSystem) {
     Y_ABORT_UNLESS(actorSystem);
     TGuard<TMutex> g(Mutex);
@@ -1456,9 +1460,8 @@ std::future<void> TMon::Start(TActorSystem* actorSystem) {
 }
 
 void TMon::Stop() {
-    IndexMonPage->ClearPages(); // it's required to avoid loop-reference
+    TGuard<TMutex> g(Mutex);
     if (ActorSystem) {
-        TGuard<TMutex> g(Mutex);
         for (const auto& [path, actorId] : ActorServices) {
             ActorSystem->Send(actorId, new TEvents::TEvPoisonPill);
         }

ydb/core/mon/mon.h

@@ -42,7 +42,7 @@ class TMon {
     };
 
     TMon(TConfig config);
-    virtual ~TMon() = default;
+    virtual ~TMon();
 
     std::future<void> Start(TActorSystem* actorSystem); // signals when monitoring is ready
     void Stop();