Core/DBLayer: Add QueueSize() method (#26733)

* Core/DBLayer: Add QueueSize() method

Add QueueSize() method to database objects (Login, Character and World) that returns how many tasks are queued.
Include the queue size of the 3 databases in ".server debug" command

* Make the code less threadsafe

* Send data to InfluxDB

* Update grafana dashboard

(cherry picked from commit 96dc110f4a)

src/common/Threading/ProducerConsumerQueue.h

@@ -52,6 +52,11 @@ public:
         return _queue.empty();
     }
 
+    size_t Size() const
+    {
+        return _queue.size();
+    }
+
     bool Pop(T& value)
     {
         std::lock_guard<std::mutex> lock(_queueLock);