Method handler

test/beast/http/write.cpp:632–632

        static std::atomic<std::size_t>&
        count() { static std::atomic<std::size_t> n; return n; }
        handler() { ++count(); }
        ~handler() { --count(); }
        handler(handler const&) { ++count(); }
        void operator()(error_code const&, std::size_t) const {}