Add a histogram to log the size of an UMA upload.

This is better than the existing pair of histograms
UMA.ProtoCompressionRatio and UMA.ProtoGzippedKBSaved
which require manual calculation to get at the approx.
value.

BUG=409179

Review URL: https://codereview.chromium.org/523153002

Cr-Commit-Position: refs/heads/master@{#293011}

components/metrics/metrics_service.cc

@@ -1026,7 +1026,9 @@ void MetricsService::OnLogUploadComplete(int response_code) {
   // Provide boolean for error recovery (allow us to ignore response_code).
   bool discard_log = false;
   const size_t log_size = log_manager_.staged_log().length();
-  if (!upload_succeeded && log_size > kUploadLogAvoidRetransmitSize) {
+  if (upload_succeeded) {
+    UMA_HISTOGRAM_COUNTS_10000("UMA.LogSize.OnSuccess", log_size / 1024);
+  } else if (log_size > kUploadLogAvoidRetransmitSize) {
     UMA_HISTOGRAM_COUNTS("UMA.Large Rejected Log was Discarded",
                          static_cast<int>(log_size));
     discard_log = true;

tools/metrics/histograms/histograms.xml

@@ -34648,6 +34648,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="UMA.LogSize.OnSuccess" units="KB">
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Size in kilobytes (after compression) of an uploaded UMA log. Recorded after
+    a successful UMA upload.
+  </summary>
+</histogram>
+
 <histogram name="UMA.LowEntropySourceValue">
   <owner>asvitkine@chromium.org</owner>
   <summary>