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/tests/meson.build b/tests/meson.build
index 5c21d88..70de004 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -27,6 +27,7 @@
             '../src/trigger_actions.cpp',
             '../src/trigger_factory.cpp',
             '../src/trigger_manager.cpp',
+            '../src/types/readings.cpp',
             '../src/types/report_types.cpp',
             '../src/utils/conversion_trigger.cpp',
             '../src/utils/generate_id.cpp',