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/test/meson.build b/test/meson.build
index 4df0d46..f240bc0 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -36,3 +36,21 @@
         include_directories: '../',
     )
 )
+
+test(
+    'test_health_metric',
+    executable(
+        'test_health_metric',
+        'test_health_metric.cpp',
+        '../health_metric.cpp',
+        '../health_utils.cpp',
+        dependencies: [
+            gtest_dep,
+            gmock_dep,
+            phosphor_logging_dep,
+            phosphor_dbus_interfaces_dep,
+            sdbusplus_dep
+        ],
+        include_directories: '../',
+    )
+)