health: Use average value for metrics dbus value

In https://gbmc-review.git.corp.google.com/c/meta-gbmc-staging/+/45155,
we added the change to use average value for threshold... however, it
also changed the metric dbus value to use instantaneous value.

Tested:
- Platform5: http://fusion2/a7d4b885-8193-3222-927b-a5ad7c86721b
- Platform11:
  - http://fusion2/4b0eda88-3a0f-37d6-8670-0cf9d18e79e9
  - http://fusion2/33e0ae4c-6606-3770-9b1a-f9863ed9897d
- Platform15: http://fusion2/16ee84a1-b837-3591-bfcc-9e83cda12df8
- Platform17: http://fusion2/843869a6-dbeb-3f8d-bd5f-463e9ab04470
Platforms-Affected: all TTF
Google-Bug-Id: 425363873
Change-Id: Ic8e901eed76e4b6abb2f76de158e7bbb27a23db2
Signed-off-by: Willy Tu <wltu@google.com>
(cherry picked from commit fc9a13231f4eb0935791e5877b2e2a7c976f1cd2)
(cherry picked from commit 3900d3d5030d07fd7ad2f557dfb821a3862ed4b6)