Added Periodic reportingType support to Report

When report interval expires report will collect readings from
metrics and update timestamp.

Tested:
  - Added new units tests covering added code
  - All existing unit tests are passing

Change-Id: I7f23ca05d77efb0f18d2c0d0f138c524ffb4f6af
Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
diff --git a/src/metric_value.hpp b/src/metric_value.hpp
new file mode 100644
index 0000000..d01ac4e
--- /dev/null
+++ b/src/metric_value.hpp
@@ -0,0 +1,12 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+struct MetricValue
+{
+    std::string id;
+    std::string metadata;
+    double value;
+    uint64_t timestamp;
+};