increment counter every time we overrid a message in async mode.

include/spdlog/details/circular_q.h

@@ -31,6 +31,7 @@ public:
         if (tail_ == head_) // overrun last item if full
         {
             head_ = (head_ + 1) % max_items_;
+            ++overrun_counter_;
         }
     }
 
@@ -53,12 +54,19 @@ public:
         return ((tail_ + 1) % max_items_) == head_;
     }
 
+    int overrun_counter() const
+    {
+      return overrun_counter_;
+    }
+
 private:
     size_t max_items_;
     typename std::vector<T>::size_type head_ = 0;
     typename std::vector<T>::size_type tail_ = 0;
 
     std::vector<T> v_;
+
+    int overrun_counter_ = 0;
 };
 } // namespace details
 } // namespace spdlog

include/spdlog/details/mpmc_blocking_q.h

@@ -30,6 +30,11 @@ public:
     {
     }
 
+    int overrun_counter() const
+    {
+      return q_.overrun_counter();
+    }
+
 #ifndef __MINGW32__
     // try to enqueue and block if no room left
     void enqueue(T &&item)

include/spdlog/details/thread_pool.h

@@ -157,6 +157,11 @@ public:
         post_async_msg_(async_msg(std::move(worker_ptr), async_msg_type::flush), overflow_policy);
     }
 
+    int overrun_counter() const
+    {
+      return q_.overrun_counter();
+    }
+
 private:
     q_type q_;