Certificate Transparency: Collect metrics on age of SCT vs STH

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 297bd54f7a948120210ab76bb17abc9a899c2440..57c3ba7029f7acad08a50e5221a59421cef1730f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26569,6 +26569,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.CertificateTransparency.CanInclusionCheckSCT"
+    enum="BooleanSCTCanBeChecked">
+  <owner>eranm@chromium.org</owner>
+  <summary>
+    Whether an observed Signed Certificate Timestamp (SCT) is covered by the
+    Signed Tree Head (STH) currently known to this client, meaning the timestamp
+    from the SCT is less than the timestamp from the STH + the log's Maximum
+    Merge Delay (24 hours). In that case it can be checked for inclusion
+    immediately without waiting for a new STH.
+  </summary>
+</histogram>
+
 <histogram name="Net.CertificateTransparency.MainFrameValidSCTCount">
   <owner>eranm@chromium.org</owner>
   <summary>
@@ -67826,6 +67838,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Revoked"/>
 </enum>
 
+<enum name="BooleanSCTCanBeChecked" type="int">
+  <int value="0" label="Requires newer STH"/>
+  <int value="1" label="Can be checked for inclusion"/>
+</enum>
+
 <enum name="BooleanSecure" type="int">
   <int value="0" label="Insecure"/>
   <int value="1" label="Secure"/>