Add descriptions for WebRTC probing histograms.

This CL adds descriptions for the histograms added in
https://webrtc-review.googlesource.com/c/src/+/127884

Bug: webrtc:10413
Change-Id: I37adf4a909242aa41b159a5454284a24aded6e5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538426
Commit-Queue: Jonas Olsson <jonasolsson@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644396}

tools/metrics/histograms/histograms.xml

@@ -132405,6 +132405,56 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.BWE.Probing.ProbeClusterSizeInBytes" units="bytes"
+    expires_after="2020-10-01">
+  <owner>jonasolsson@chromium.org</owner>
+  <owner>crodbro@chromium.org</owner>
+  <summary>
+    The size in bytes of the probe cluster. Counted for each cluster once all
+    its packets have been sent.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.BWE.Probing.ProbesPerCluster"
+    expires_after="2020-10-01">
+  <owner>jonasolsson@chromium.org</owner>
+  <owner>crodbro@chromium.org</owner>
+  <summary>
+    The size in packets of the probe cluster. Counted for each cluster once all
+    its packets have been sent.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.BWE.Probing.TimePerProbeCluster" units="ms"
+    expires_after="2020-10-01">
+  <owner>jonasolsson@chromium.org</owner>
+  <owner>crodbro@chromium.org</owner>
+  <summary>
+    The time from sending the first to the last packet of the probe cluster.
+    Counted for each cluster once all its packets have been sent.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.BWE.Probing.TotalFailedProbeClusters"
+    expires_after="2020-10-01">
+  <owner>jonasolsson@chromium.org</owner>
+  <owner>crodbro@chromium.org</owner>
+  <summary>
+    Counts the amount of probe clusters that timed out. Counted when the prober
+    is destroyed.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.BWE.Probing.TotalProbeClustersRequested"
+    expires_after="2020-10-01">
+  <owner>jonasolsson@chromium.org</owner>
+  <owner>crodbro@chromium.org</owner>
+  <summary>
+    Counts the amount of probe clusters that the bitrate prober created. Counted
+    when the prober is destroyed.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.BWE.RampUpTimeTo1000kbpsInMs" units="ms">
   <owner>holmer@chromium.org</owner>
   <summary>