Create FieldTrialAllocator.Size histogram

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index 911c8e5ee9842759c3c820929e268098e6a6dbd6..2fd0bd2e9ce6957be8b42afde536eeda4ff901e8 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -12,6 +12,7 @@
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/logging.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/process/memory.h"
 #include "base/rand_util.h"
 #include "base/strings/string_number_conversions.h"
@@ -724,6 +725,8 @@ void FieldTrialList::CopyFieldTrialStateToFlags(
                                      std::to_string(field_trial_length);
 
     cmd_line->AppendSwitchASCII(field_trial_handle_switch, field_trial_handle);
+    UMA_HISTOGRAM_COUNTS_10000("UMA.Metrics.FieldTrialAllocator.Size",
+                               field_trial_length);
     return;
   }
 #endif