[metrics] Record size of dropped logs

Unsent metrics logs get trimmed if they're larger than a preset size.
This change implements a new UMA metrics histogram called
"UMA.UnsentLogs.DroppedSize" to record the size of each of these trimmed
logs.

Also adds a unit test case for a similar existing histogram that
measures dropped log counts.

Bug: None
Test: metrics_unittests
Change-Id: If6ed17a2a1327c2bd3d3a9da16926b91e72a6f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482322
Commit-Queue: Akira Baruah <akirabaruah@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819434}

components/metrics/unsent_log_store_metrics_impl.cc

@@ -15,14 +15,16 @@ void UnsentLogStoreMetricsImpl::RecordLogReadStatus(
                                 UnsentLogStoreMetrics::END_RECALL_STATUS);
 }
 
-void UnsentLogStoreMetricsImpl::RecordCompressionRatio(
-    size_t compressed_size, size_t original_size) {
+void UnsentLogStoreMetricsImpl::RecordCompressionRatio(size_t compressed_size,
+                                                       size_t original_size) {
   base::UmaHistogramPercentageObsoleteDoNotUse(
       "UMA.ProtoCompressionRatio",
       static_cast<int>(100 * compressed_size / original_size));
 }
 
 void UnsentLogStoreMetricsImpl::RecordDroppedLogSize(size_t size) {
+  base::UmaHistogramCounts1M("UMA.UnsentLogs.DroppedSize",
+                             static_cast<int>(size));
 }
 
 void UnsentLogStoreMetricsImpl::RecordDroppedLogsNum(int dropped_logs_num) {

components/metrics/unsent_log_store_metrics_impl_unittest.cc

@@ -10,6 +10,22 @@
 
 namespace metrics {
 
+TEST(UnsentLogStoreMetricsImplTest, RecordDroppedLogSize) {
+  UnsentLogStoreMetricsImpl impl;
+  base::HistogramTester histogram_tester;
+
+  impl.RecordDroppedLogSize(99999);
+  histogram_tester.ExpectBucketCount("UMA.UnsentLogs.DroppedSize", 99999, 1);
+}
+
+TEST(UnsentLogStoreMetricsImplTest, RecordDroppedLogsNum) {
+  UnsentLogStoreMetricsImpl impl;
+  base::HistogramTester histogram_tester;
+
+  impl.RecordDroppedLogsNum(17);
+  histogram_tester.ExpectBucketCount("UMA.UnsentLogs.Dropped", 17, 1);
+}
+
 TEST(UnsentLogStoreMetricsImplTest, RecordLastUnsentLogMetadataMetrics) {
   base::test::ScopedFeatureList feature_override;
   feature_override.InitAndEnableFeature(

tools/metrics/histograms/histograms_xml/uma/histograms.xml

@@ -590,6 +590,20 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="UMA.UnsentLogs.DroppedSize" units="bytes"
+    expires_after="2021-08-30">
+  <owner>akirabaruah@chromium.org</owner>
+  <owner>rkaplow@chromium.org</owner>
+  <owner>asvitkine@chromium.org</owner>
+  <owner>src/base/metrics/OWNERS</owner>
+  <summary>
+    Size of unsent logs removed from persistent storage due to exceeding the
+    individual log size limit. Unsent log trimming occurs immediately after
+    initial logs creation, upon successful log upload, or whenever
+    MetricsService stops recording.
+  </summary>
+</histogram>
+
 <histogram name="UMA.UnsentLogs.PersistedSizeInKB" units="KB"
     expires_after="2021-05-30">
   <owner>michaelbai@chromium.org</owner>