add health_metric implementation

Add the interface and implementation for the health_metric to be used
in the rewrite for phosphor-health-monitor.
This change is in relation to following design and D-Bus interface
update -
https://gerrit.openbmc.org/c/openbmc/docs/+/64917
https://gerrit.openbmc.org/c/openbmc/phosphor-dbus-interfaces/+/64914

gtest added for UT.

Change-Id: Iffcc52f9dff712890377b1222fd7e7d5d6661eaf
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index 27e2bff..1abe217 100644
--- a/meson.build
+++ b/meson.build
@@ -26,6 +26,8 @@
     [
         'healthMonitor.cpp',
         'health_metric_config.cpp',
+        'health_metric.cpp',
+        'health_utils.cpp',
     ],
     dependencies: [
         base_deps