Add polymorphism support to Ref

src/metric.cpp

@@ -444,7 +444,7 @@ struct Metric {
   }
 
   static auto &get_collectors() {
-    using CollectorRegistry = std::vector<std::shared_ptr<Collector>>;
+    using CollectorRegistry = std::vector<Ref<Collector>>;
     static CollectorRegistry *collectors = new CollectorRegistry();
     return *collectors;
   }
@@ -1803,7 +1803,7 @@ void reset_metrics_for_testing() {
   // when threads exit naturally
 }
 
-void register_collector(std::shared_ptr<Collector> collector) {
+void register_collector(Ref<Collector> collector) {
   std::unique_lock<std::mutex> _{Metric::mutex};
   ++Metric::registration_version;
   Metric::get_collectors().push_back(std::move(collector));