Implement permission embargo suppression metrics.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 6bf2c915e8f644ad7c823b3aaa1ef5dc45b998a8..499e5a91760ffb764172e606b5ad6e832c5072a5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -46390,6 +46390,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Permissions.AutoBlocker.EmbargoPromptSuppression"
+    enum="PermissionEmbargoStatus">
+  <owner>dominickn@chromium.org</owner>
+  <owner>kcarattini@chromium.org</owner>
+  <summary>
+    For every permission request that would show a prompt to the user, this
+    metric tracks whether the request is suppressed by embargo (and the source
+    of that embargo), or whether there is no embargo and the prompt is shown.
+    This is the proportion of permission prompts that the user would have seen
+    that have been blocked due to embargo.
+
+    If the (origin, permission) pair was previously placed under embargo, no
+    prompt is displayed and the reason for embargo is recorded. Otherwise, the
+    prompt is shown and a no embargo reason is recorded.
+  </summary>
+</histogram>
+
 <histogram name="Permissions.AutoBlocker.EmbargoStatus"
     enum="PermissionEmbargoStatus">
   <owner>dominickn@chromium.org</owner>