message: Fix value type of histogram bucket counters
diff: @@ -28,13 +28,11 @@ public:
  using gauge_type = gauge<value_type>;
  - using counter_type = counter<value_type>;
  -
  using family_setting = std::vector<value_type>;
  struct bucket_type {
  value_type upper_bound;
  - counter_type count;
  + int_counter count;
  };
  // -- constants --------------------------------------------------------------
  