Measure the X-Chrome-Variations construction time with UMA.

chrome/browser/metrics/variations/variations_http_header_provider.cc

 // Copyright (c) 2012 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/metrics/variations/variations_http_header_provider.h"
 
 #include "base/base64.h"
 #include "base/memory/singleton.h"
+#include "base/metrics/histogram.h"
 #include "chrome/browser/google/google_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_io_data.h"
 #include "chrome/common/metrics/proto/chrome_experiments.pb.h"
 #include "chrome/common/metrics/variations/variations_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/http/http_request_headers.h"
 
 using content::BrowserThread;
 
@@ skipping 66 matching lines @@
 void VariationsHttpHeaderProvider::InitVariationIDsCacheIfNeeded() {
   base::AutoLock scoped_lock(lock_);
   if (variation_ids_cache_initialized_)
     return;
 
   // Register for additional cache updates. This is done first to avoid a race
   // that could cause registered FieldTrials to be missed.
   DCHECK(MessageLoop::current());
   base::FieldTrialList::AddObserver(this);
 
+  base::TimeTicks before_time = base::TimeTicks::Now();
+
   base::FieldTrial::ActiveGroups initial_groups;
   base::FieldTrialList::GetActiveFieldTrialGroups(&initial_groups);
   for (base::FieldTrial::ActiveGroups::const_iterator it =
            initial_groups.begin();
        it != initial_groups.end(); ++it) {
     const VariationID id =
         GetGoogleVariationID(GOOGLE_WEB_PROPERTIES, it->trial_name,
                              it->group_name);
     if (id != EMPTY_ID)
       variation_ids_set_.insert(id);
   }
   UpdateVariationIDsHeaderValue();
 
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "Variations.HeaderConstructionTime",
+      (base::TimeTicks::Now() - before_time).InMicroseconds(),
+      0,
+      base::TimeDelta::FromSeconds(1).InMicroseconds(),
+      50);
+
   variation_ids_cache_initialized_ = true;
 }
 
 void VariationsHttpHeaderProvider::UpdateVariationIDsHeaderValue() {
   lock_.AssertAcquired();
 
   // The header value is a serialized protobuffer of Variation IDs which is
   // base64 encoded before transmitting as a string.
   if (variation_ids_set_.empty())
     return;
@@ skipping 23 matching lines @@
     // if IDs are added as the header is recreated. The receiving servers are OK
     // with such descrepancies.
     variation_ids_header_ = hashed;
   } else {
     NOTREACHED() << "Failed to base64 encode Variation IDs value: "
                  << serialized;
   }
 }
 
 }  // namespace chrome_variations