[NTP Snippets] Add metric for fetch error cases

components/ntp_snippets/ntp_snippets_fetcher.cc

Index: components/ntp_snippets/ntp_snippets_fetcher.cc
diff --git a/components/ntp_snippets/ntp_snippets_fetcher.cc b/components/ntp_snippets/ntp_snippets_fetcher.cc
index 7fe3d94b0e54b22c86e052357ff1bf1e2a1f0f48..007d337e65a2a2b0aa8f6187d3b85b5b60d10bf0 100644
--- a/components/ntp_snippets/ntp_snippets_fetcher.cc
+++ b/components/ntp_snippets/ntp_snippets_fetcher.cc
@@ -6,6 +6,7 @@
 
 #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"
@@ -120,12 +121,23 @@ void NTPSnippetsFetcher::OnURLFetchComplete(const URLFetcher* 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;