made MetricValues persistent

MetricValues are persistent for reportUpdates AppendStopsWhenFull and
reportingType different than OnRequest.

Tested:
- New unit tests are passing
- Confirmed MetricValues are preserved after restarting telemetry
  service

Change-Id: I7e1990fb391da9debb0d7df2f1dbda86473350cc
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/src/utils/labeled_tuple.hpp b/src/utils/labeled_tuple.hpp
index 71bf4b0..e6203dd 100644
--- a/src/utils/labeled_tuple.hpp
+++ b/src/utils/labeled_tuple.hpp
@@ -76,6 +76,11 @@
         return j;
     }
 
+    const tuple_type& to_tuple() const
+    {
+        return value;
+    }
+
     void from_json(const nlohmann::json& j)
     {
         from_json_all(j, std::make_index_sequence<sizeof...(Args)>());