Use Arena's to manage Metric memory where appropriate

src/metric.hpp

@@ -36,7 +36,6 @@
 //   histogram.observe(0.25);  // ONLY call from creating thread
 
 #include <functional>
-#include <initializer_list>
 #include <span>
 #include <string>
 #include <type_traits>
@@ -197,4 +196,4 @@ bool is_valid_label_value(const std::string &value);
 // Note: Histograms do not support callbacks due to their multi-value nature
 // (buckets + sum + count). Use static histogram metrics only.
 
-} // namespace metric
+} // namespace metric