Measure and log the latency of VariationsService requests.

chrome/browser/metrics/variations/variations_service.cc

Index: chrome/browser/metrics/variations/variations_service.cc
diff --git a/chrome/browser/metrics/variations/variations_service.cc b/chrome/browser/metrics/variations/variations_service.cc
index 87a4ed954a70370fbae8d7821860044d97a8eb2c..ec8b00594476dfec8199d18963b024bc76e7c522 100644
--- a/chrome/browser/metrics/variations/variations_service.cc
+++ b/chrome/browser/metrics/variations/variations_service.cc
@@ -96,6 +96,15 @@ GURL GetVariationsServerURL() {
   return url_as_gurl;
 }
 
+// Logs |latency| in the time histogram named |name|.
+void LogRequestLatency(const char* name, base::TimeDelta latency) {
+  UMA_HISTOGRAM_CUSTOM_TIMES(name,

[Ilya Sherman, 2012/10/02 21:09:33]

This won't work.  The name passed to any of the UMA_HISTOGRAM macros needs to be
a fixed string during the entire run of Chrome, as the macro internally caches
the histogram in a static variable.

[Ilya Sherman, 2012/10/02 21:09:33]

nit: Why do we need custom times for this histogram, rather than using one of
MEDIUM_ or LONG_TIMES?

[SteveT, 2012/10/03 19:51:34]

Darn, okay. It was odd that my testing seemed to work.

I've reverted to calling UMA_HISTOGRAMS_... three times.

[SteveT, 2012/10/03 19:51:34]

I was following what how some Net code was measuring some Http request
latencies... But it looked like that code was just for the handshake.

3 minutes seems like a reasonable upper bound on this histogram, so I'll use
MEDIUM_TIMES unless you think otherwise.

+      latency,
+      base::TimeDelta::FromMilliseconds(1),
+      base::TimeDelta::FromMinutes(10),
+      100);
+}
+
 }  // namespace
 
 VariationsService::VariationsService()
@@ -183,6 +192,8 @@ void VariationsService::DoActualFetch() {
                                                  variations_serial_number_);
   }
   pending_seed_request_->Start();
+
+  last_request_started_time_ = base::TimeTicks::Now();
 }
 
 void VariationsService::FetchVariationsSeed() {
@@ -211,11 +222,19 @@ void VariationsService::OnURLFetchComplete(const net::URLFetcher* source) {
       net::HttpUtil::MapStatusCodeForHistogram(request->GetResponseCode()),
       net::HttpUtil::GetStatusCodesForHistogram());
 
+  const base::TimeDelta latency =
+      base::TimeTicks::Now() - last_request_started_time_;
+
   if (request->GetResponseCode() != 200) {
     DVLOG(1) << "Variations server request returned non-200 response code: "
              << request->GetResponseCode();
+    if (request->GetResponseCode() == 304)
+      LogRequestLatency("Variations.FetchNotModifiedLatency", latency);
+    else
+      LogRequestLatency("Variations.FetchOtherLatency", latency);
     return;
   }
+  LogRequestLatency("Variations.FetchSuccessLatency", latency);
 
   std::string seed_data;
   bool success = request->GetResponseAsString(&seed_data);