Add a histogram to measure the frequency of OCSP stapling support by servers

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 540e482ac84d1b24d1a5c6210eb366f54ce1d12c..b98183f1d5068dce35f7dd7d2a0bc9a0da72ae69 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9267,6 +9267,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Net.OCSPResponseStapled" enum="BooleanSuccess">

[jar (doing other things), 2014/02/19 22:29:16]

This is ok.... but sometimes the histogram is more readable if you create your
own custom enum, with choices like:

No stapling found
Contains stapled OCSP

In this case, your boolean might be clear enough as as.  Again, your choice.

+  <summary>
+    If OCSP stapling was requested (eg: a platform that supports OCSP stapling
+    or OCSP stapling was requested for Certificate Transparency), indicates the
+    server included a stapled OCSP response.
+  </summary>
+</histogram>
+
 <histogram name="Net.OSErrorsForGetAddrinfo" enum="ErrorCodesGetaddrinfo_All">
   <summary>
     Positive error code that was returned by the system library