[ChromeOS.PlatformKeysService Histograms] Extend expiration time

By looking at [1] & [2], it looks like no users are currently generating
EC keys or signing using them. Let's extend the expiration time for this
metric for another 6 months to continue monitoring their usage.

[1] https://uma.googleplex.com/histograms?sid=19825cd3aaa4b39a242c48f52f489639
[2] https://uma.googleplex.com/histograms?sid=5449d558cc544dd9f5bad6d90c1056bf

Bug: 1237990, 1237991, b/197612402, b/197612483
Change-Id: I3044db675fa2d0edf0f9bd2899cd0a6ba75be580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3114449
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Andreea-Elena Costinas <acostinas@google.com>
Commit-Queue: Omar Morsi <omorsi@google.com>
Cr-Commit-Position: refs/heads/main@{#915077}

tools/metrics/histograms/metadata/chromeos/histograms.xml

@@ -925,7 +925,7 @@ incoming reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="ChromeOS.PlatformKeysService.{Operation}.{KeyType}"
-    enum="BooleanSuccess" expires_after="2021-09-19">
+    enum="BooleanSuccess" expires_after="2022-03-19">
   <owner>omorsi@google.com</owner>
   <owner>mgawad@google.com</owner>
   <summary>