Update expiration for OSCrypt.BackendUsage

This metric tracks usage of backends and is useful long-term

Bug: 1140055
Change-Id: Ie672922dd8d465da30472fec947582c216ef46f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540611Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828268}

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -9870,7 +9870,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="OSCrypt.BackendUsage" enum="LinuxPasswordStoreUsage"
-    expires_after="2020-12-01">
+    expires_after="2021-07-01">
   <owner>cfroussios@chromium.org</owner>
   <owner>mamir@chromium.org</owner>
   <summary>