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

chrome/browser/metrics/variations/variations_http_header_provider.cc

Index: chrome/browser/metrics/variations/variations_http_header_provider.cc
diff --git a/chrome/browser/metrics/variations/variations_http_header_provider.cc b/chrome/browser/metrics/variations/variations_http_header_provider.cc
index 1e7d7877cf3242a34d1e5fbf2e958076538b2728..be823bd4acb63b4a5e604179d15dd76313f67574 100644
--- a/chrome/browser/metrics/variations/variations_http_header_provider.cc
+++ b/chrome/browser/metrics/variations/variations_http_header_provider.cc
@@ -6,6 +6,7 @@
 
 #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"
@@ -92,6 +93,8 @@ void VariationsHttpHeaderProvider::InitVariationIDsCacheIfNeeded() {
   DCHECK(MessageLoop::current());
   base::FieldTrialList::AddObserver(this);
 
+  base::TimeTicks before_time = base::TimeTicks::HighResNow();
+
   base::FieldTrial::ActiveGroups initial_groups;
   base::FieldTrialList::GetActiveFieldTrialGroups(&initial_groups);
   for (base::FieldTrial::ActiveGroups::const_iterator it =
@@ -105,6 +108,13 @@ void VariationsHttpHeaderProvider::InitVariationIDsCacheIfNeeded() {
   }
   UpdateVariationIDsHeaderValue();
 
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "Variations.HeaderConstructionTime",
+      (base::TimeTicks::HighResNow() - before_time).InMicroseconds(),

[jar (doing other things), 2013/02/01 00:50:37]

Since you're converting to microseconds, don't bother with HighResNow(), just
use TimeTicks::Now() here, and on line 96.

We'll get an "average" with lots of zeros, and that will tend to allow us to see
a slightly higher res at the low end (if this is consistently under 1ms).  In
addition, if it is way under 1ms, we shouldn't worry much about it in the field.
 If it is larger... then we'll see clearly... and we'll have something to fret
about.

[SteveT, 2013/02/01 14:17:45]

That sounds good. Thanks for pointing it out.

Done.

+      0,
+      base::TimeDelta::FromSeconds(1).InMicroseconds(),
+      50);
+
   variation_ids_cache_initialized_ = true;
 }