health metric: check hysteresis for value notify

Add hysteresis for metric value and check it before notifying the metric
value update.

Change-Id: I8dace4b9436dd9b16f1a075505079d5001928f52
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/health_metric.hpp b/health_metric.hpp
index 9584953..cf2f66b 100644
--- a/health_metric.hpp
+++ b/health_metric.hpp
@@ -62,6 +62,9 @@
     void create(const paths_t& bmcPaths);
     /** @brief Init properties for the health metric object */
     void initProperties();
+    /** @brief Check if specified value should be notified based on hysteresis
+     */
+    auto shouldNotify(MValue value) -> bool;
     /** @brief Check specified threshold for the given value */
     void checkThreshold(Type type, Bound bound, MValue value);
     /** @brief Check all thresholds for the given value */
@@ -76,6 +79,8 @@
     const config::HealthMetric config;
     /** @brief Window for metric history */
     std::deque<double> history;
+    /** @brief Last notified value for the metric change */
+    double lastNotifiedValue = 0;
 };
 
 } // namespace phosphor::health::metric