QUIC - add new UMA histograms to help understand HPACK head of line blocking.

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 30624972a915710a4c2637c2b8a0ebe493a6f5a5..bab0f583dc4af5879fdf91ea2b2d56999310bdfe 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -28427,6 +28427,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicHpackDecoder.IndexedEntryAge" units="ms">
+  <owner>ckrasic@chromium.org</owner>
+  <summary>
+    For each indexed representation decoded, records the time since the
+    corresponding entry was added to the dynamic table.  This data is being
+    collected to help analyze a proposed solution to HPACK induced head of line
+    blocking.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicHpackDecompressionPercentage" units="%">
   <owner>ckrasic@chromium.org</owner>
   <summary>
@@ -28435,6 +28445,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicHpackEncoder.IndexedEntryAge" units="ms">
+  <owner>ckrasic@chromium.org</owner>
+  <summary>
+    For each indexed representation encoded, records the time since the
+    corresponding entry was added to the dynamic table.  This data is being
+    collected to help analyze a proposed solution to HPACK induced head of line
+    blocking.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicInchoateClientHelloReason"
     enum="QuicServerConfigState">
   <owner>rtenneti@chromium.org</owner>