Measure per metric in render scale bench

src/metric.cpp

@@ -1695,30 +1695,26 @@ void reset_metrics_for_testing() {
   std::lock_guard _{Metric::mutex};
   ++Metric::registration_version;
 
-  // WARNING: This function assumes no metric objects are in use!
-  // Clear all family maps - this will leak the Family::State objects but
-  // that's acceptable for testing since they were allocated in the global arena
+  Metric::get_counter_families().clear();
+  Metric::get_gauge_families().clear();
+  Metric::get_histogram_families().clear();
+  Metric::get_interned_labels().clear();
+  Metric::get_interned_static_text().clear();
 
-  // Get references to the maps
-  auto &counter_families = Metric::get_counter_families();
-  auto &gauge_families = Metric::get_gauge_families();
-  auto &histogram_families = Metric::get_histogram_families();
-  auto &interned_labels = Metric::get_interned_labels();
-
-  // Clear all family registrations
-  counter_families.clear();
-  gauge_families.clear();
-  histogram_families.clear();
-  interned_labels.clear();
-
-  // Clear interned static text
-  auto &interned_static_text = Metric::get_interned_static_text();
-  interned_static_text.clear();
-
-  // Reset the global arena - this will invalidate all arena-allocated strings
-  // but since we're clearing everything, that's OK
   Metric::get_global_arena().reset();
 
+  auto reset_arena = [](auto &m) {
+    using M = std::decay_t<decltype(m)>;
+    using A = M::allocator_type;
+    m = M{A{&Metric::get_global_arena()}};
+  };
+
+  reset_arena(Metric::get_counter_families());
+  reset_arena(Metric::get_gauge_families());
+  reset_arena(Metric::get_histogram_families());
+  reset_arena(Metric::get_interned_labels());
+  reset_arena(Metric::get_interned_static_text());
+
   // Note: Thread-local arenas will be cleaned up by ThreadInit destructors
   // when threads exit naturally
 }