{

public:

/// Overloads operator() that sums numeric values

template <

typename T,

typename = typename std::enable_if<std::is_arithmetic<T>::value, T>::type>

std::variant<int, std::string, double, uint64_t> operator()(const T& a, const T& b) const

{

return a + b;

}

/// If arguments have different type an exception is raised

/// \throws MonitoringException

template <typename T, typename U>

std::variant<int, std::string, double, uint64_t> operator()(const T&, const U&) const

{

throw MonitoringException("DerivedMetrics/Visitor", "Cannot operate on different or non-numeric types");

}