Add getConnectionCount method (#1731)

lib/inc/drogon/HttpAppFramework.h

@@ -1521,6 +1521,10 @@ class DROGON_EXPORT HttpAppFramework : public trantor::NonCopyable
 
     virtual HttpAppFramework &enableCompressedRequest(bool enable = true) = 0;
     virtual bool isCompressedRequestEnabled() const = 0;
+    /*
+     * @brief get the number of active connections.
+     */
+    virtual int64_t getConnectionCount() const = 0;
 
   private:
     virtual void registerHttpController(

lib/src/HttpAppFrameworkImpl.h

@@ -606,6 +606,11 @@ class HttpAppFrameworkImpl final : public HttpAppFramework
         const std::string &ext,
         const std::string &mime) override;
 
+    int64_t getConnectionCount() const override
+    {
+        return connectionNum_.load(std::memory_order_relaxed);
+    }
+
   private:
     void registerHttpController(const std::string &pathPattern,
                                 const internal::HttpBinderBasePtr &binder,