Add polymorphism support to Ref

src/metric.hpp

@@ -51,6 +51,7 @@
 #include <vector>
 
 #include "arena.hpp"
+#include "reference.hpp"
 
 namespace metric {
 
@@ -255,12 +256,12 @@ struct Collector {
 /**
  * @brief Register a collector with the metrics system.
  *
- * The system will hold a shared_ptr to the collector and call its collect()
+ * The system will hold a Ref to the collector and call its collect()
  * method during each metric rendering.
  *
- * @param collector A shared_ptr to the collector to be registered.
+ * @param collector A Ref to the collector to be registered.
  */
-void register_collector(std::shared_ptr<Collector> collector);
+void register_collector(Ref<Collector> collector);
 
 // Note: Histograms do not support callbacks due to their multi-value nature
 // (buckets + sum + count). Use static histogram metrics only.