sysmon: Fix yet another typo

This is the third typo I have had to fix in this bit of copy-pasted
code.

BUG=chromium:736911
BUG=chromium:737146
BUG=chromium:737217
TEST=None

Change-Id: I14ed0337c64c58bc4868aff2a338eddc9a51eb3d
Reviewed-on: https://chromium-review.googlesource.com/553458
Reviewed-by: Paul Hobbs <phobbs@google.com>
Tested-by: Allen Li <ayatane@chromium.org>
diff --git a/scripts/sysmon/git_metrics.py b/scripts/sysmon/git_metrics.py
index 46ee62f9..bcf8dea 100644
--- a/scripts/sysmon/git_metrics.py
+++ b/scripts/sysmon/git_metrics.py
@@ -104,9 +104,9 @@
 
   def _collect_unstaged_changes_metric(self):
     added, deleted = self._gitrepo.get_unstaged_changes()
-    self._timestamp_metric.set(
+    self._unstaged_changes_metric.set(
         added, fields=dict(change_type='added', **self._fields))
-    self._timestamp_metric.set(
+    self._unstaged_changes_metric.set(
         deleted, fields=dict(change_type='deleted', **self._fields))