Handle multiple modifications to distribution metrics correctly.

infra_libs/ts_mon/common/metrics.py

 # Copyright 2015 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.
 
 """Classes representing individual metrics that can be sent."""
 
 import copy
 import operator
 import threading
 import time
@@ skipping 175 matching lines @@
     It might be easier to call ts_mon.reset_for_unittest() in your setUp()
     method instead of resetting every individual metric.
     """
 
     interface.state.store.reset_for_unittest(self.name)
 
   def _set(self, fields, value, enforce_ge=False):
     interface.state.store.set(self.name, self._normalize_fields(fields), value,
                               enforce_ge=enforce_ge)
 
-  def _incr(self, fields, delta, modify_fn=operator.add):
+  def _incr(self, fields, delta, modify_fn=None):
     interface.state.store.incr(self.name, self._normalize_fields(fields), delta,
                                modify_fn=modify_fn)
 
 
 class StringMetric(Metric):
   """A metric whose value type is a string."""
 
   def _populate_value(self, metric, value, start_time):
     metric.string_value = value
 
@@ skipping 211 matching lines @@
   """A DistributionMetric with is_cumulative set to False."""
 
   def __init__(self, name, bucketer=None, fields=None,
                description=None):
     super(NonCumulativeDistributionMetric, self).__init__(
         name,
         is_cumulative=False,
         bucketer=bucketer,
         fields=fields,
         description=description)