diff --git a/src/utils/labeled_tuple.hpp b/src/utils/labeled_tuple.hpp
index 795f37f..a224141 100644
--- a/src/utils/labeled_tuple.hpp
+++ b/src/utils/labeled_tuple.hpp
@@ -170,15 +170,15 @@
 };
 
 template <class... Args, class... Labels>
-void to_json(nlohmann::json& json,
-             const LabeledTuple<std::tuple<Args...>, Labels...>& tuple)
+inline void to_json(nlohmann::json& json,
+                    const LabeledTuple<std::tuple<Args...>, Labels...>& tuple)
 {
     json = tuple.to_json();
 }
 
 template <class... Args, class... Labels>
-void from_json(const nlohmann::json& json,
-               LabeledTuple<std::tuple<Args...>, Labels...>& tuple)
+inline void from_json(const nlohmann::json& json,
+                      LabeledTuple<std::tuple<Args...>, Labels...>& tuple)
 {
     tuple.from_json(json);
 }
diff --git a/src/utils/tstring.hpp b/src/utils/tstring.hpp
index a8d3e90..3e85ba5 100644
--- a/src/utils/tstring.hpp
+++ b/src/utils/tstring.hpp
@@ -56,5 +56,21 @@
     }
 };
 
+struct CollectionTimeScope
+{
+    static std::string str()
+    {
+        return "collectionTimeScope";
+    }
+};
+
+struct CollectionDuration
+{
+    static std::string str()
+    {
+        return "collectionDuration";
+    }
+};
+
 } // namespace tstring
 } // namespace utils