MSE: Record counts of detected MSE audio, video and text tracks

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 b42f34c6a2b2dd16b1070c699d0f2899c5dab4dd..df0b11e05ec0f4fd16420ed7ec9c5aeaf0ceae83 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20369,6 +20369,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.MSE.DetectedTrackCount.Audio">
+  <owner>wolenetz@chromium.org</owner>
+  <summary>
+    Number of detected audio tracks in Media Source Extensions playback. Not all
+    may be usable by the player.
+  </summary>
+</histogram>
+
+<histogram name="Media.MSE.DetectedTrackCount.Text">
+  <owner>wolenetz@chromium.org</owner>
+  <summary>
+    Number of detected text tracks in Media Source Extensions playback. Not all
+    may be usable by the player. This count includes only explicitly signalled
+    tracks in MSE initialization segments parsed from WebM or ISO BMFF
+    bytestreams.
+  </summary>
+</histogram>
+
+<histogram name="Media.MSE.DetectedTrackCount.Video">
+  <owner>wolenetz@chromium.org</owner>
+  <summary>
+    Number of detected video tracks in Media Source Extensions playback. Not all
+    may be usable by the player.
+  </summary>
+</histogram>
+
 <histogram name="Media.MSE.LateAudioFrames"
     units="late frames per million frames">
   <owner>qinmin@chromium.org</owner>