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/src/interfaces/metric.hpp b/src/interfaces/metric.hpp
index 50bf5d5..d63c979 100644
--- a/src/interfaces/metric.hpp
+++ b/src/interfaces/metric.hpp
@@ -14,6 +14,7 @@
   public:
     virtual ~Metric() = default;
 
+    virtual void initialize() = 0;
     virtual const std::vector<MetricValue>& getReadings() const = 0;
     virtual nlohmann::json to_json() const = 0;
 };