Basic UkmService implementation

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 984189dfd2c1d1afb7c26880c322108798b3bc7a..02c8c6978b191debb302dcae82e03b472d4d5e19 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -68602,6 +68602,77 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UKM.ActualLogUploadInterval" units="minutes">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    The actual interval between log upload start and previous log upload
+    finished within the same process.
+  </summary>
+</histogram>
+
+<histogram name="UKM.InitSequence" enum="UmaInitSequence">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    Logged during UkmService initialization whether the init task or the initial
+    log timer completed first. The expectation is the vast majority of the time,
+    the init task should complete first. If metrics show otherwise, then it may
+    indicate there's a bug in the UkmService init sequence and that it should be
+    investigated.
+  </summary>
+</histogram>
+
+<histogram name="UKM.LogSize.OnSuccess" units="KB">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    Size in kilobytes (after compression) of an uploaded UKM log. Recorded after
+    a successful UKM upload.
+  </summary>
+</histogram>
+
+<histogram name="UKM.OversizeRejected.LogSize" units="bytes">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    Number of bytes in a log was was rejected by server, and then discarded.
+  </summary>
+</histogram>
+
+<histogram name="UKM.PersistentLogRecall.Status"
+    enum="PersistedLogsLogReadStatus">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>The status when loading UKM PersistedLogs from Prefs.</summary>
+</histogram>
+
+<histogram name="UKM.UnsentLogs.DroppedSize" units="bytes">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    Number of bytes in an excessively large log that was discarded while
+    persisting UKM logs to disk to retry during next chrome run.
+  </summary>
+</histogram>
+
+<histogram name="UKM.UnsentLogs.NumDropped">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    Counter for number of UKM unsent logs removed from persistent storage.
+  </summary>
+</histogram>
+
+<histogram name="UKM.Upload.ResponseCode" enum="HttpResponseCode">
+  <owner>holte@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    For each upload to the UKM server, log the response received from the
+    server.
+  </summary>
+</histogram>
+
 <histogram name="UMA.ActualLogUploadInterval" units="minutes">
   <owner>asvitkine@chromium.org</owner>
   <summary>