Extend contextual nudge histograms by one year.

Bug: 1147178
Change-Id: I0daed330aa98fee3166372f98c693cbef6ab5a69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527843
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825652}

tools/metrics/histograms/histograms_xml/ash/histograms.xml

@@ -349,25 +349,31 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram base="true" name="Ash.ContextualNudgeDismissContext"
-    enum="ContextualNudgeDismissContext" expires_after="M86">
+    enum="ContextualNudgeDismissContext" expires_after="2021-12-01">
 <!-- Name completed by histogram_suffixes name="ContextualNudgesNames" -->
 
   <owner>yulunwu@chromium.org</owner>
   <owner>tbarzic@chromium.org</owner>
   <summary>
     Tracks the reason contextual nudges for user gesture education are
-    dismissed.
+    dismissed. Metrics are recorded when the user performs an action that
+    renders the shown contextual nudge irrelevant. Metrics are recorded when the
+    user performs the gesture suggested by the nudge as well as when the user
+    exits the relevant state some other way.
   </summary>
 </histogram>
 
 <histogram base="true" name="Ash.ContextualNudgeDismissTime" units="ms"
-    expires_after="M86">
+    expires_after="2021-12-01">
 <!-- Name completed by histogram_suffixes name="ContextualNudgesNames" -->
 
   <owner>yulunwu@chromium.org</owner>
   <owner>tbarzic@chromium.org</owner>
   <summary>
     Tracks time between showing contextual nudge and gesture being performed.
+    Metrics are only recorded when the user performs the action suggested by the
+    contextual nudge. No metrics are recorded if the user exits the relevant
+    state some other way.
   </summary>
 </histogram>