Add missing types to counting_iterator

include/fmt/format.h

@@ -628,6 +628,12 @@ class counting_iterator {
   mutable T blackhole_;
 
  public:
+  using iterator_category = std::output_iterator_tag;
+  using value_type = T;
+  using difference_type = std::ptrdiff_t;
+  using pointer = T*;
+  using reference = T&;
+
   explicit counting_iterator(std::size_t &count): count_(count) {}
   counting_iterator(const counting_iterator &other): count_(other.count_) {}