Created metric class

Metric collects updates from sensor. Report displays metric readings
depending on reportingType.

Tested:
  - Added new units tests for Metric class
  - All other unit tests are passing

Change-Id: I19f4831fab163a4f9540cef7bb23e903ae90fddf
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/tests/src/mocks/metric_mock.hpp b/tests/src/mocks/metric_mock.hpp
index d3d9e64..21f7733 100644
--- a/tests/src/mocks/metric_mock.hpp
+++ b/tests/src/mocks/metric_mock.hpp
@@ -15,6 +15,7 @@
             .WillByDefault(ReturnRefOfCopy(std::vector<MetricValue>()));
     }
 
+    MOCK_METHOD(void, initialize, (), (override));
     MOCK_METHOD(const std::vector<MetricValue>&, getReadings, (),
                 (const, override));
     MOCK_METHOD(nlohmann::json, to_json, (), (const, override));