Use Arena's to manage Metric memory where appropriate

src/arena_allocator.hpp

@@ -637,6 +637,14 @@ template <typename T> struct ArenaVector {
   T &operator[](size_t index) { return data_[index]; }
   const T &operator[](size_t index) const { return data_[index]; }
 
+  void clear() { size_ = 0; }
+
+  // Iterator support for range-based for loops
+  T *begin() { return data_; }
+  const T *begin() const { return data_; }
+  T *end() { return data_ + size_; }
+  const T *end() const { return data_ + size_; }
+
   // No destructor - arena cleanup handles memory
 
 private: