Add metrics to evaluate number of undecryptable sync entities

This CL adds a histogram that records, per data type, what was the
number of sync entities that remained undecrypted when it was disabled.

Bug: 1109221
Change-Id: I4b97f99286a85f3f5bd4f3ee4cf8de206ff93d69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2326710
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793168}

components/sync/engine_impl/model_type_worker.cc

@@ -14,6 +14,7 @@
 #include "base/format_macros.h"
 #include "base/guid.h"
 #include "base/logging.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_restrictions.h"
@@ -108,6 +109,10 @@ ModelTypeWorker::ModelTypeWorker(
 }
 
 ModelTypeWorker::~ModelTypeWorker() {
+  base::UmaHistogramCounts1000(
+      std::string("Sync.UndecryptedEntitiesOnDataTypeDisabled.") +
+          ModelTypeToHistogramSuffix(type_),
+      entries_pending_decryption_.size());
   model_type_processor_->DisconnectSync();
 }
 

tools/metrics/histograms/histograms.xml

@@ -175953,6 +175953,16 @@ should be kept until we use this API.  -->
   </summary>
 </histogram>
 
+<histogram base="true" name="Sync.UndecryptedEntitiesOnDataTypeDisabled"
+    units="sync entities" expires_after="2021-01-30">
+  <owner>victorvianna@google.com</owner>
+  <owner>treib@chromium.org</owner>
+  <summary>
+    This records the number of sync entities that were still undecrypted by the
+    time the data type was disabled (typically during browser shutdown).
+  </summary>
+</histogram>
+
 <histogram name="Sync.UnrecoverableErrors" enum="SyncUnrecoverableErrorReason"
     expires_after="M85">
   <obsolete>
@@ -215421,6 +215431,7 @@ regressions. -->
       Deprecated 05/2019.
     </obsolete>
   </affected-histogram>
+  <affected-histogram name="Sync.UndecryptedEntitiesOnDataTypeDisabled"/>
   <affected-histogram name="Sync.USSMigrationEntityCount">
     <obsolete>
       Deprecated 05/2020.