[time] Adds the 'Started from Backstop' metric

This metric allows us to track the UTC clocks that have been
started from the backstop time.

Starting the UTC clock is required for some faulty legacy
programs that assume that the UTC clock is monotonic.

Bug: b/295537827
Change-Id: If156b311b2696d6a7c3a1a4e2e63c9308dcba14c
Reviewed-on: https://fuchsia-review.googlesource.com/c/cobalt-registry/+/906828
Fuchsia-Auto-Submit: Filip Filmar <fmil@google.com>
Privacy-Approval: Steve Fung <stevefung@google.com>
Reviewed-by: Steve Fung <stevefung@google.com>
Commit-Queue: Filip Filmar <fmil@google.com>

fuchsia/time/metrics.yaml

diff --git a/fuchsia/time/metrics.yaml b/fuchsia/time/metrics.yaml
index b1c0ba7..a3b67bb 100644
--- a/fuchsia/time/metrics.yaml
+++ b/fuchsia/time/metrics.yaml
@@ -146,6 +146,9 @@
         6: WriteFailed
         # A write to the RTC succeeded.
         7: WriteSucceeded
+        # The UTC clock is started from backstop time. This supports continuous
+        # UTC operation.
+        8: StartedFromBackstop
       max_event_code: 15
   reports:
     - report_name: counts