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/types/readings.hpp b/src/types/readings.hpp
new file mode 100644
index 0000000..6749bbf
--- /dev/null
+++ b/src/types/readings.hpp
@@ -0,0 +1,24 @@
+#pragma once
+
+#include "utils/labeled_tuple.hpp"
+#include "utils/tstring.hpp"
+
+using ReadingData = std::tuple<std::string, std::string, double, uint64_t>;
+using Readings = std::tuple<uint64_t, std::vector<ReadingData>>;
+
+using LabeledReadingData =
+    utils::LabeledTuple<ReadingData, utils::tstring::MetricId,
+                        utils::tstring::MetricProperty,
+                        utils::tstring::MetricValue, utils::tstring::Timestamp>;
+
+using LabeledReadings =
+    utils::LabeledTuple<std::tuple<uint64_t, std::vector<LabeledReadingData>>,
+                        utils::tstring::Timestamp, utils::tstring::Readings>;
+
+namespace utils
+{
+
+LabeledReadings toLabeledReadings(const Readings&);
+Readings toReadings(const LabeledReadings&);
+
+} // namespace utils