Include what is used

Author: jerryct

core/include/prometheus/registry.h

@@ -6,8 +6,10 @@
 
 #include "prometheus/client_metric.h"
 #include "prometheus/collectable.h"
+#include "prometheus/counter.h"
 #include "prometheus/counter_builder.h"
 #include "prometheus/family.h"
+#include "prometheus/gauge.h"
 #include "prometheus/gauge_builder.h"
 #include "prometheus/histogram.h"
 #include "prometheus/histogram_builder.h"
@@ -40,4 +42,4 @@ class Registry : public Collectable {
   std::vector<std::unique_ptr<Collectable>> collectables_;
   std::mutex mutex_;
 };
-}
+}  // namespace prometheus