Add method to cound number of subscription

Adafruit_MQTT.cpp

@@ -890,6 +890,18 @@ uint8_t Adafruit_MQTT::disconnectPacket(uint8_t *packet) {
   return 2;
 }
 
+uint Adafruit_MQTT::getSubscriptionCount()
+{
+  uint count = 0;
+
+  for (int i = 0; i < MAXSUBSCRIPTIONS; i++) {
+    if (subscriptions[i])
+      count++;
+  }
+
+  return count;
+}
+
 // Adafruit_MQTT_Publish Definition ////////////////////////////////////////////
 
 Adafruit_MQTT_Publish::Adafruit_MQTT_Publish(Adafruit_MQTT *mqttserver,

Adafruit_MQTT.h

@@ -218,6 +218,9 @@ public:
   // Ping the server to ensure the connection is still alive.
   bool ping(uint8_t n = 1);
 
+  // Count the number of registrered Adafruit_MQTT_Subscribe
+  uint getSubscriptionCount();
+
 protected:
   // Interface that subclasses need to implement: