update_engine: Add new metric to AttemptResult

Add more granularity to metric |AttemptResult| to avoid most error
metrics falling into the category |kInternalError|.

BUG=chromium:1086956
TEST=cros_workon_make update_engine --test

Change-Id: I37a4b55543011cc2eb5ced38cebf14d5794e9482
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2314898
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
Tested-by: Andrew Lassalle <andrewlassalle@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/payload_state.cc b/payload_state.cc
index f227026..b6c054b 100644
--- a/payload_state.cc
+++ b/payload_state.cc
@@ -672,6 +672,7 @@
     case metrics::AttemptResult::kAbnormalTermination:
     case metrics::AttemptResult::kUpdateCanceled:
     case metrics::AttemptResult::kUpdateSucceededNotActive:
+    case metrics::AttemptResult::kUpdateSkipped:
     case metrics::AttemptResult::kNumConstants:
     case metrics::AttemptResult::kUnset:
       break;