added support for onChange report

Report is now notified when metric changes and updates reading values.

Tested:
  - Added new unit tests
  - OnChange report updates Readings when metric values changes

Change-Id: I3be9ef7aa0486cb15bac627aa1de5cc632613b3b
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/tests/meson.build b/tests/meson.build
index 89c6a00..5c21d88 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -11,9 +11,10 @@
     executable(
         'telemetry-ut',
         [
-            '../src/details/collection_function.cpp',
             '../src/discrete_threshold.cpp',
             '../src/metric.cpp',
+            '../src/metrics/collection_data.cpp',
+            '../src/metrics/collection_function.cpp',
             '../src/numeric_threshold.cpp',
             '../src/on_change_threshold.cpp',
             '../src/persistent_json_storage.cpp',
@@ -36,6 +37,7 @@
             'src/test_conversion.cpp',
             'src/test_detached_timer.cpp',
             'src/test_discrete_threshold.cpp',
+            'src/test_ensure.cpp',
             'src/test_generate_id.cpp',
             'src/test_metric.cpp',
             'src/test_numeric_threshold.cpp',