[NTP Snippets] Add metric for fetch error cases

components/ntp_snippets/ntp_snippets_fetcher.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 "components/ntp_snippets/ntp_snippets_fetcher.h"
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
+#include "base/metrics/sparse_histogram.h"
 #include "base/path_service.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/task_runner_util.h"
 #include "google_apis/google_api_keys.h"
 #include "net/base/load_flags.h"
 #include "net/http/http_request_headers.h"
 #include "net/http/http_response_headers.h"
 #include "net/http/http_status_code.h"
@@ skipping 94 matching lines @@
   url_fetcher_->Start();
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // URLFetcherDelegate overrides
 void NTPSnippetsFetcher::OnURLFetchComplete(const URLFetcher* source) {
   DCHECK_EQ(url_fetcher_.get(), source);
 
   std::string message;
   const URLRequestStatus& status = source->GetStatus();
+
   if (!status.is_success()) {
+    // Error codes are guaranteed to be negative so let's negate to use positive
+    // buckets.
+    UMA_HISTOGRAM_SPARSE_SLOWLY("NewTabPage.Snippets.FailedRequestErrorCode",
+                                -status.error());
+
     message = base::StringPrintf(kStatusMessageURLRequestErrorFormat,
                                  status.error());
-  } else if (source->GetResponseCode() != net::HTTP_OK) {
-    message = base::StringPrintf(kStatusMessageHTTPErrorFormat,
-                                 source->GetResponseCode());
+  } else {
+    UMA_HISTOGRAM_SPARSE_SLOWLY("NewTabPage.Snippets.FetchResponseCode",

[Alexei Svitkine (slow), 2016/05/02 17:15:14]

You can actually log these two together, see CombinedHttpResponseAndNetErrorCode
enum in histograms.xml. Since error codes are negative (which are supported by
sparse histograms) and response codes are positive, this works just fine.

+                                source->GetResponseCode());
+
+    if (source->GetResponseCode() != net::HTTP_OK) {
+      message = base::StringPrintf(kStatusMessageHTTPErrorFormat,
+                                   source->GetResponseCode());
+    }
   }
 
   std::string response;
   if (!message.empty()) {
     DLOG(WARNING) << message << " while trying to download "
                   << source->GetURL().spec();
 
   } else {
     bool stores_result_to_string = source->GetResponseAsString(&response);
     DCHECK(stores_result_to_string);
   }
 
   callback_list_.Notify(response, message);
 }
 
 }  // namespace ntp_snippets