Add a comment clarifying how UkmEntryBuilder::AddMetric works.

components/ukm/ukm_entry_builder.h

 // Copyright 2017 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.
 
 #ifndef COMPONENTS_UKM_UKM_ENTRY_BUILDER_H
 #define COMPONENTS_UKM_UKM_ENTRY_BUILDER_H
 
 #include <string>
 
 #include "base/macros.h"
@@ skipping 14 matching lines @@
 //   builder->AddMetric("ResponseStart", response_start_time);
 //   builder->AddMetric("FirstPaint", first_paint_time);
 //   builder->AddMetric("FirstContentfulPaint", fcp_time);
 // }
 //
 // When there exists an added metric, the builder will automatically add the
 // UkmEntry to UkmService upon destruction when going out of scope.
 class UkmEntryBuilder {
  public:
   // Add metric to the entry. A metric contains a metric name and value.
+  // Repeated calls with the same metric name will add multiple values,
+  // rather than replace the existing value.
   void AddMetric(const char* metric_name, int64_t value);
 
   ~UkmEntryBuilder();
 
  private:
   friend class UkmService;
 
   UkmEntryBuilder(const UkmService::AddEntryCallback& callback,
                   int32_t source_id,
                   const char* event_name);
 
   UkmService::AddEntryCallback add_entry_callback_;
   std::unique_ptr<UkmEntry> entry_;
 
   DISALLOW_COPY_AND_ASSIGN(UkmEntryBuilder);
 };
 
 }  // namespace ukm
 
 #endif  // COMPONENTS_UKM_UKM_ENTRY_BUILDER_H