Finish metrics design, I think

src/metric.hpp

@@ -142,6 +142,10 @@ Family<Histogram> create_histogram(std::string name, std::string help,
                                    std::initializer_list<double> buckets);
 
 // Render all metrics in Prometheus text format
+// 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
 std::span<std::string_view> render(ArenaAllocator &arena);