Add a comment clarifying how UkmEntryBuilder::AddMetric works.

components/ukm/ukm_entry_builder.h

Index: components/ukm/ukm_entry_builder.h
diff --git a/components/ukm/ukm_entry_builder.h b/components/ukm/ukm_entry_builder.h
index e2400ee5cb5afe47e7022ad343735a6a0dc58225..5a2284eea186f93f63999038d0bcaa5709ed3aa6 100644
--- a/components/ukm/ukm_entry_builder.h
+++ b/components/ukm/ukm_entry_builder.h
@@ -32,6 +32,8 @@ class UkmService;
 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();