Add UMA histograms for Chrome OS ML Service

Add histograms for the following metrics added in Chrome OS CL
crrev.com/c/1209022.
* Numerical histograms for CPU and RAM usage of ML Service
* Enum histogram for Mojo connection/disconnection events

Bug: 881186
Change-Id: Id649c3bce16bf6ca65871feee1ab741372530669
Reviewed-on: https://chromium-review.googlesource.com/1237017Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593465}

tools/metrics/histograms/enums.xml

@@ -30749,6 +30749,12 @@ from previous Chrome versions.
   <int value="22" label="MacBookPro5,X"/>
 </enum>
 
+<enum name="MachineLearningServiceMojoConnectionEvent">
+  <int value="0" label="Mojo bootstrap requested over D-Bus"/>
+  <int value="1" label="Mojo bootstrap succeeded"/>
+  <int value="2" label="Connection error"/>
+</enum>
+
 <enum name="MachineLevelUserCloudPolicyEnrollmentResult">
   <int value="0" label="Success"/>
   <int value="1" label="Failed to fetch DM Token from the server"/>

tools/metrics/histograms/histograms.xml

@@ -40438,6 +40438,42 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="MachineLearningService.CpuUsageMilliPercent"
+    units="1/1000ths of %" expires_after="2019-03-01">
+  <owner>amoylan@chromium.org</owner>
+  <summary>
+    Fraction of total CPU resources used by Chrome OS ML Service, sampled every
+    5 minutes.
+  </summary>
+</histogram>
+
+<histogram name="MachineLearningService.MojoConnectionEvent"
+    enum="MachineLearningServiceMojoConnectionEvent" expires_after="2019-03-01">
+  <owner>amoylan@chromium.org</owner>
+  <summary>
+    Events related to the connection and disconnection of the Mojo IPC channel
+    between Chrome and the Chrome OS ML Service.
+  </summary>
+</histogram>
+
+<histogram name="MachineLearningService.PeakPrivateMemoryKb" units="KB"
+    expires_after="2019-03-01">
+  <owner>amoylan@chromium.org</owner>
+  <summary>
+    Peak private (non-shared) memory used by Chrome OS ML Service over the last
+    24 hours. Sampled every 5 minutes, so may miss short-lived spikes.
+  </summary>
+</histogram>
+
+<histogram name="MachineLearningService.PrivateMemoryKb" units="KB"
+    expires_after="2019-03-01">
+  <owner>amoylan@chromium.org</owner>
+  <summary>
+    Private (non-shared) memory used by Chrome OS ML Service, sampled every 5
+    minutes.
+  </summary>
+</histogram>
+
 <histogram name="ManagedUsers.ChromeOS.PasswordChange"
     enum="ManagedUserPasswordChange">
   <owner>achuith@chromium.org</owner>