login: Add UMA metric for continue-arc-boot impulse

Add "Login.ArcContinueBootImpulseTime" metric to track the
time taken to execute continue-arc-boot impulse.

See https://crrev.com/c/2512710 for code that emits the metric

BUG=b:165173640
TEST=deploy to dut and verify the new metric is present in

chrome: //histograms
Change-Id: I6dee096fc1665f402f4adfb898fd1dc56ddc86d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512724
Commit-Queue: Muhammad Hasan Khan <mhasank@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823317}

tools/metrics/histograms/histograms_xml/login/histograms.xml

@@ -22,11 +22,22 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 <histograms>
 
 <histogram name="Login.ArcBugReportBackupTime" units="ms"
-    expires_after="2021-04-04">
+    expires_after="2021-10-30">
+  <owner>mhasank@google.com</owner>
+  <owner>arc-core@google.com</owner>
+  <summary>
+    Tracks the time to execute arc-boot-continue impulse. This is emitted when
+    we fail to upgrade the ARC container from mini to full.
+  </summary>
+</histogram>
+
+<histogram name="Login.ArcContinueBootImpulseTime" units="ms"
+    expires_after="2021-10-30">
   <owner>mhasank@google.com</owner>
   <owner>arc-core@google.com</owner>
   <summary>
-    Tracks the time to backup ARC bug report when starting ARC fails.
+    Tracks the time to backup ARC bug report when starting ARC fails. This is
+    emitted when we upgrade the ARC container from mini to full.
   </summary>
 </histogram>