Add metrics for SameParty exclusions on cookie reads and writes.

These metrics are only updated for excluded cookies, and their values
are whether or not SameParty was the only reason for excluding the
cookie.

Bug: 1143756
Change-Id: Icc35c59d47269e89f8f37515a7a86d6dc33d0591
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611046Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Lily Chen <chlily@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841113}

net/cookies/cookie_monster.cc

@@ -981,6 +981,11 @@ void CookieMonster::FilterCookiesWithOptions(
             cookie_util::GetSamePartyStatus(*cookie_ptr, options)});
 
     if (!access_result.status.IsInclude()) {
+      UMA_HISTOGRAM_BOOLEAN(
+          "Cookie.SameParty.ReadExclusionDecidedBySameParty",
+          access_result.status.HasOnlyExclusionReason(
+              CookieInclusionStatus::EXCLUDE_SAMEPARTY_CROSS_PARTY_CONTEXT));
+
       if (options.return_excluded_cookies())
         excluded_cookies->push_back({*cookie_ptr, access_result});
       continue;
@@ -1305,6 +1310,11 @@ void CookieMonster::SetCanonicalCookie(std::unique_ptr<CanonicalCookie> cc,
           "Cookie.Port.Set.RemoteHost",
           ReducePortRangeForCookieHistogram(source_url.EffectiveIntPort()));
     }
+  } else {
+    UMA_HISTOGRAM_BOOLEAN(
+        "Cookie.SameParty.SetExclusionDecidedBySameParty",
+        access_result.status.HasOnlyExclusionReason(
+            CookieInclusionStatus::EXCLUDE_SAMEPARTY_CROSS_PARTY_CONTEXT));
   }
 
   // TODO(chlily): Log metrics.

tools/metrics/histograms/histograms_xml/cookie/histograms.xml

@@ -387,6 +387,26 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Cookie.SameParty.ReadExclusionDecidedBySameParty"
+    enum="Boolean" expires_after="2022-01-06">
+  <owner>cfredric@chromium.org</owner>
+  <owner>chlily@chromium.org</owner>
+  <summary>
+    This histogram records whether enforcement of the SameParty attribute was
+    the only reason for a cookie's exclusion (from a read).
+  </summary>
+</histogram>
+
+<histogram name="Cookie.SameParty.SetExclusionDecidedBySameParty"
+    enum="Boolean" expires_after="2022-01-06">
+  <owner>cfredric@chromium.org</owner>
+  <owner>chlily@chromium.org</owner>
+  <summary>
+    This histogram records whether enforcement of the SameParty attribute was
+    the only reason for a cookie's exclusion (from a write).
+  </summary>
+</histogram>
+
 <histogram name="Cookie.SamePartyCookieSameSiteAttributeValue"
     enum="CookieSameSiteString" expires_after="2021-11-01">
   <owner>chlily@chromium.org</owner>