Improve metric.hpp documentation

src/metric.hpp

@@ -164,7 +164,6 @@ private:
 };
 
 // Factory functions for creating metric families
-// IMPORTANT: name and help must point to static memory (string literals)
 
 // Create counter family (monotonically increasing values)
 // ERROR: Aborts if family with same name is registered with different help
@@ -205,9 +204,9 @@ std::vector<double> exponential_buckets(double start, double factor, int count);
 // Returns chunks of Prometheus exposition format (includes # HELP and # TYPE
 // lines) Each string_view may contain multiple lines separated by '\n' String
 // views are NOT null-terminated - use .size() for length All string data
-// allocated in provided arena for zero-copy efficiency
-// TODO: Implement Prometheus text exposition format
-// THREAD SAFETY: Serialized by global mutex - callbacks need not be thread-safe
+// allocated in provided arena for zero-copy efficiency. The caller is
+// responsible for the arena's lifecycle. THREAD SAFETY: Serialized by global
+// mutex - callbacks need not be thread-safe
 std::span<std::string_view> render(ArenaAllocator &arena);
 
 // Validation functions for Prometheus compatibility