Add histogram for comparing link monitors

The new added metric shows the time diff for detecting an L2
connectivity issue between the two link monitors.

CrOS patch to emit the metric: https://crrev.com/c/2439961

BUG=b:162194516
TEST=See the TEST line in CrOS patch

Change-Id: I9b1cb421f2d0c0accd3e151d006cad254d4289a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462753
Commit-Queue: Jie Jiang <jiejiang@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Hugo Benichi <hugobenichi@google.com>
Cr-Commit-Position: refs/heads/master@{#834098}

tools/metrics/histograms/histograms_xml/network/histograms.xml

@@ -1325,6 +1325,38 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Network.Shill.Wifi.LinkMonitorsDetectionTimeDiff.ArpBetter"
+    units="ms" expires_after="M91">
+  <owner>jiejiang@chromium.org</owner>
+  <owner>cros-network-metrics@google.com</owner>
+  <summary>
+    Chrome OS metric that tracks the number of milliseconds between the
+    detection times for the same failure of NeighborLinkMonitor and
+    shill::LinkMonitor. Recorded after either of the link monitor detects an
+    error. The larger this value is, the better shill::LinkMonitor performs than
+    NeighborLinkMonitor. 0 means shill::LinkMonitor is no better than
+    NeighborLinkMonitor. If shill::LinkMonitor detects a failure but
+    NeighborLinkMonitor does not, the maximum value of a timeout will be
+    emitted.
+  </summary>
+</histogram>
+
+<histogram
+    name="Network.Shill.Wifi.LinkMonitorsDetectionTimeDiff.NeighborBetter"
+    units="ms" expires_after="M91">
+  <owner>jiejiang@chromium.org</owner>
+  <owner>cros-network-metrics@google.com</owner>
+  <summary>
+    Chrome OS metric that tracks the number of milliseconds between the
+    detection times for the same failure of patchpanel::NeighborLinkMonitor and
+    shill::LinkMonitor. Recorded after either of the link monitor detects an
+    error. The larger this value is, the better NeighborLinkMonitor performs
+    than shill::LinkMonitor. 0 means NeighborLinkMonitor is no better than
+    shill::LinkMonitor. If NeighborLinkMonitor detects a failure but
+    shill::LinkMonitor does not, the maxmium value of a timeout will be emitted.
+  </summary>
+</histogram>
+
 <histogram name="Network.Shill.Wifi.LinkMonitorSecondsToFailure"
     units="seconds" expires_after="2021-12-01">
   <owner>briannorris@chromium.org</owner>