Extend SignedExchange related histograms expiration date

We still want to preserve the following histograms:
* PrefetchedSignedExchangeCache.HeadersSizeTotal
* Net.NetworkErrorLogging.SignedExchangeRequestOutcome

Bug: 1165258, 1165256
Change-Id: If7d302b8298e89a1d2a164b8f81a459725e1431b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626617
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843023}

tools/metrics/histograms/histograms_xml/net/histograms.xml

@@ -1920,7 +1920,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Net.NetworkErrorLogging.SignedExchangeRequestOutcome"
-    enum="NetNetworkErrorLoggingRequestOutcome" expires_after="2021-01-24">
+    enum="NetNetworkErrorLoggingRequestOutcome" expires_after="M94">
   <owner>horo@chromium.org</owner>
   <owner>ksakamoto@chromium.org</owner>
   <owner>kinuko@chromium.org</owner>

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

@@ -11677,7 +11677,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="PrefetchedSignedExchangeCache.HeadersSizeTotal" units="bytes"
-    expires_after="2021-01-24">
+    expires_after="M94">
   <owner>horo@chromium.org</owner>
   <owner>webpackage-dev@chromium.org</owner>
   <summary>