Finish metrics design, I think

src/metric.cpp

@@ -76,15 +76,7 @@ struct LabelsKey {
 namespace std {
 template <> struct hash<metric::LabelsKey> {
   std::size_t operator()(const metric::LabelsKey &k) const {
-    thread_local std::vector<size_t> parts;
+    return std::hash<decltype(k.labels)>{}(k.labels);
-    parts.clear();
-    for (const auto &p : k.labels) {
-      parts.push_back(std::hash<std::string>{}(p.first));
-      parts.push_back(std::hash<std::string>{}(p.second));
-    }
-    return std::hash<std::string>{}(
-        std::string{reinterpret_cast<const char *>(parts.data()),
-                    parts.size() * sizeof(size_t)});
   }
 };
 } // namespace std

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);