My issue was caused because of course I made a mistake with the way of registering my metrics Listener.

+ MyMetrics.RegisterListener(IDTAG.c_str(), "*", std::bind(&canlog::MetricListener, this, _1));

and for the destructor:

+ MyMetrics.DeregisterListener(IDTAG.c_str());

I had to use IDTAG because this class has multiple instances.

The issue is passing IDTAG.c_str() as a parameter ; because MyMetrics.RegisterListener stores the value of this const char* parameter and uses it 1) as an immutable parameter and 2) as a value.

Of course this does not work with my first way to call MyMetrics.RegisterListener :

the memory buffer returned by IDTAG.c_str() has no more existence after exiting the constructor, so this lead to memory corruption.
when calling IDTAG.c_str() again in the destructor, there is absolutely no chance that this c_str() call returns the same memory address as the call in the constructor, so the value comparison will never work.

I rewrote the code this way by adding a new member (m_idtag) to the class:

@ vehicle/OVMS.V3/components/can/src/canlog.cpp:352 @ canlog::canlog(const char* type, std::string format, canformat::canformat_serve_
   m_dropcount = 0;
   m_filtercount = 0;

+ m_idtag = strdup(IDTAG.c_str());
+
   using std::placeholders::_1;
   using std::placeholders::_2;
   MyEvents.RegisterEvent(IDTAG, "*", std::bind(&canlog::EventListener, this, _1, _2));
+ if (NULL != m_idtag)
+    {
+    MyMetrics.RegisterListener(m_idtag, "*", std::bind(&canlog::MetricListener, this, _1));
+    }
+ else
+    {
+    ESP_LOGE(TAG,"Cannot allocate memory - not registering metrics listener");
+    }

   int queuesize = MyConfig.GetParamValueInt("can", "log.queuesize",100);
   m_queue = xQueueCreate(queuesize, sizeof(CAN_log_message_t));
@ vehicle/OVMS.V3/components/can/src/canlog.cpp:374 @ canlog::canlog(const char* type, std::string format, canformat::canformat_serve_
canlog::~canlog()
   {
   MyEvents.DeregisterEvent(IDTAG);
+ if (NULL != m_idtag)
+    {
+    MyMetrics.DeregisterListener(m_idtag);
+    free((void*)m_idtag);
+    }

   if (m_task)
     {

Another option is to change MyMetrics.RegisterListener so that it store *a copy* of the parameter, and does string comparison instead of a value comparison:

modified: vehicle/OVMS.V3/main/ovms_metrics.cpp
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
@ vehicle/OVMS.V3/main/ovms_metrics.cpp:1019 @ void OvmsMetrics::RegisterListener(const char* caller, const char* name, MetricC
     return;
     }

+ const char * caller_copy = (const char *)strdup(caller);
+
+ if (NULL == caller_copy)
+    {
+    ESP_LOGE(TAG, "Problem registering metric %s for caller %s - insufficient memory",name,caller);
+    return;
+    }
+
   MetricCallbackList *ml = k->second;
- ml->push_back(new MetricCallbackEntry(caller,callback));
+ ml->push_back(new MetricCallbackEntry(caller_copy,callback));
   }

void OvmsMetrics::DeregisterListener(const char* caller)
@ vehicle/OVMS.V3/main/ovms_metrics.cpp:1041 @ void OvmsMetrics::DeregisterListener(const char* caller)
     while (itc!=ml->end())
       {
       MetricCallbackEntry* ec = *itc;
-      if (ec->m_caller == caller)
+      if (strcmp(ec->m_caller, caller) == 0)
         {
+        free((char *)ec->m_caller);
         itc = ml->erase(itc);
         delete ec;
         }