[instance] introduce ActiveInstanceTracker for context-aware logging (#12869)

This commit introduces `ActiveInstanceTracker` as the first member
variable of the `Instance` class to manage the global `gActiveInstance`
pointer (when `OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE` is
enabled).

By placing it as the very first member, we ensure its constructor
is called before any other member and its destructor is called after
all others. The `Instance` destructor body also explicitly sets
`gActiveInstance = this` at its start to "claim" the context during its
own destruction. This guarantees that logs emitted during both the
initialization and destruction of an `Instance` are always correctly
associated with that instance. Finally, the `ActiveInstanceTracker`
destructor sets `gActiveInstance` to `nullptr` at the very end to
prevent any potential use of a dangling pointer.

src/core/instance/instance.cpp

@@ -78,7 +78,8 @@ LogLevel Instance::sGlobalLogLevel = static_cast<LogLevel>(OPENTHREAD_CONFIG_LOG
 #endif
 
 Instance::Instance(void)
-    : mTimerMilliScheduler(*this)
+    : mActiveInstanceTracker(*this)
+    , mTimerMilliScheduler(*this)
 #if OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE
     , mTimerMicroScheduler(*this)
 #endif
@@ -337,6 +338,10 @@ Instance::Instance(void)
 #endif
 }
 
+#if OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE && OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE
+Instance::~Instance(void) { gActiveInstance = this; }
+#endif
+
 #if (OPENTHREAD_MTD || OPENTHREAD_FTD) && !OPENTHREAD_CONFIG_HEAP_EXTERNAL_ENABLE
 Utils::Heap &Instance::GetHeap(void)
 {
@@ -490,16 +495,8 @@ void Instance::Finalize(void)
 
     IgnoreError(Get<Mac::SubMac>().Disable());
 
-#if !OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE
-
-    /**
-     * Object was created on buffer, so instead of deleting
-     * the object we call destructor explicitly.
-     */
     this->~Instance();
 
-#endif // !OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE
-
 exit:
     return;
 }

src/core/instance/instance.hpp

@@ -484,7 +484,41 @@ public:
     void AfterInit(void);
 #endif
 
+#if OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE && OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE
+    /**
+     * Destructor
+     */
+    ~Instance(void);
+#endif
+
 private:
+    class ActiveInstanceTracker
+    {
+    public:
+        // This class is used to track and update the `gActiveInstance` pointer during the
+        // construction and destruction of an `Instance`.
+        //
+        // `mActiveInstanceTracker` is defined as the very first member variable in `Instance`.
+        // This ensures its constructor is called before any other member components are
+        // initialized, and its destructor is called after all other components are destroyed.
+        // This ensures `gActiveInstance` is correctly set during the entire lifecycle of
+        // the `Instance`, allowing member components to safely emit logs.
+        //
+        // The `Instance` destructor also explicitly sets `gActiveInstance` to the instance
+        // being destroyed at the beginning of its body. This ensures that during the
+        // entire destruction process, log messages are correctly associated with the
+        // instance being destroyed. Finally, when `mActiveInstanceTracker` itself is
+        // destroyed (at the end of the `Instance` destruction), `gActiveInstance` is
+        // set to `nullptr` to avoid any potential use of a dangling pointer.
+
+#if OPENTHREAD_CONFIG_MULTIPLE_INSTANCE_ENABLE && OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE
+        ActiveInstanceTracker(Instance &aInstance) { gActiveInstance = &aInstance; }
+        ~ActiveInstanceTracker(void) { gActiveInstance = nullptr; }
+#else
+        ActiveInstanceTracker(Instance &) {}
+#endif
+    };
+
 #if !OPENTHREAD_PLATFORM_NEXUS
     Instance(void);
     void AfterInit(void);
@@ -509,6 +543,8 @@ private:
     // Tasklet and Timer Schedulers are first to ensure other
     // objects/classes can use them from their constructors.
 
+    ActiveInstanceTracker mActiveInstanceTracker;
+
     Tasklet::Scheduler    mTaskletScheduler;
     TimerMilli::Scheduler mTimerMilliScheduler;
 #if OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE