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/tstring.hpp b/src/utils/tstring.hpp
index e4529d8..67ad3c4 100644
--- a/src/utils/tstring.hpp
+++ b/src/utils/tstring.hpp
@@ -144,5 +144,45 @@
     }
 };
 
+struct MetricId
+{
+    static std::string str()
+    {
+        return "MetricId";
+    }
+};
+
+struct MetricProperty
+{
+    static std::string str()
+    {
+        return "MetricProperty";
+    }
+};
+
+struct MetricValue
+{
+    static std::string str()
+    {
+        return "MetricValue";
+    }
+};
+
+struct Timestamp
+{
+    static std::string str()
+    {
+        return "Timestamp";
+    }
+};
+
+struct Readings
+{
+    static std::string str()
+    {
+        return "Readings";
+    }
+};
+
 } // namespace tstring
 } // namespace utils