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;
-    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)});
+    return std::hash<decltype(k.labels)>{}(k.labels);
   }
 };
 } // namespace std