add health_metric_collection implementation

Add the interface and implementation for health_metric_collection which
encapsulates various health_metrics.
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

Add test_health_metric_collection gtest for unit testing.

Change-Id: Ia0b9fbc6bec4850735c7eb74dcd5c40fc47c568c
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index 1abe217..09ef147 100644
--- a/meson.build
+++ b/meson.build
@@ -28,6 +28,7 @@
         'health_metric_config.cpp',
         'health_metric.cpp',
         'health_utils.cpp',
+        'health_metric_collection.cpp',
     ],
     dependencies: [
         base_deps