Don't null-terminate

src/metric.cpp

@@ -128,9 +128,8 @@ static std::string_view arena_copy_string(std::string_view str,
   if (str.empty()) {
     return std::string_view{};
   }
-  char *copied = arena.allocate<char>(str.size() + 1);
+  char *copied = arena.allocate<char>(str.size());
   std::memcpy(copied, str.data(), str.size());
-  copied[str.size()] = '\0';
   return std::string_view(copied, str.size());
 }