Add Printing.CUPS.PrintersDiscovered to our metrics.

chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_util.h"
 #include "base/json/json_string_value_serializer.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "base/threading/sequenced_task_runner_handle.h"
 #include "base/values.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chromeos/printing/ppd_provider_factory.h"
 #include "chrome/browser/chromeos/printing/printer_configurer.h"
 #include "chrome/browser/chromeos/printing/printer_discoverer.h"
 #include "chrome/browser/chromeos/printing/printers_manager_factory.h"
 #include "chrome/browser/download/download_prefs.h"
@@ skipping 369 matching lines @@
   }
 
   printer_discoverer_->AddObserver(this);
 }
 
 void CupsPrintersHandler::HandleStopDiscovery(const base::ListValue* args) {
   printer_discoverer_.reset();
 }
 
 void CupsPrintersHandler::OnPrintersFound(
+    bool scan_done,
     const std::vector<Printer>& printers) {
   std::unique_ptr<base::ListValue> printers_list =
       base::MakeUnique<base::ListValue>();
   for (const auto& printer : printers) {
     printers_list->Append(GetPrinterInfo(printer));
   }
 
   FireWebUIListener("on-printer-discovered", *printers_list);
-}
 
-void CupsPrintersHandler::OnDiscoveryInitialScanDone() {

[dpapad, 2017/05/26 21:02:03]

Is it necessary to merge OnDiscoveryInitialScanDone to OnPrintersFound? Can we
avoid merging them (and the need for the  |scan_done| boolean) and adding a
boolean to OnPrintersFound by doing the following instead?

1) Keep the 2 observer methods separate.
2) Pass printers_list->GetSize() as a parameter to OnDiscoveryInitialScanDone.
3) Call UMA_HISTOGRAM_COUNTS_100() from OnDiscoveryInitialScanDone.

[skau, 2017/05/26 21:20:10]

Justin and I have been considering whether the current change or your proposed
alternative would be simpler.  We think that the change as written is a more
straightforward API independent of adding metrics.  All
OnDiscoveryInitialScanDone calls will be immediately preceded by an
OnPrintersFound call and we haven't found it useful to have a separate method.

-  FireWebUIListener("on-printer-discovery-done");
+  if (scan_done) {
+    UMA_HISTOGRAM_COUNTS_100("Printing.CUPS.PrintersDiscovered",
+                             printers_list->GetSize());
+    FireWebUIListener("on-printer-discovery-done");
+  }
 }
 
 }  // namespace settings
 }  // namespace chromeos