Avoid logging very small values for Mojo.System.MessagesAcceptedPerEvent

mojo/edk/system/node_controller.cc

Index: mojo/edk/system/node_controller.cc
diff --git a/mojo/edk/system/node_controller.cc b/mojo/edk/system/node_controller.cc
index 25a7c87efad80a139b92ddc8f9f5c4b784139218..cafd604d1cb36d78a017eee4edb3af5d3d4a33c6 100644
--- a/mojo/edk/system/node_controller.cc
+++ b/mojo/edk/system/node_controller.cc
@@ -755,11 +755,15 @@ void NodeController::AcceptIncomingMessages() {
     CHECK_LE(num_messages_accepted, kMaxAcceptedMessages);
   }
 
-  UMA_HISTOGRAM_CUSTOM_COUNTS("Mojo.System.MessagesAcceptedPerEvent",
-                              static_cast<int32_t>(num_messages_accepted),
-                              1 /* min */,
-                              500 /* max */,
-                              50 /* bucket count */);
+  if (num_messages_accepted >= 4) {
+    // Note: We avoid logging this histogram for the vast majority of cases.
+    // See https://crbug.com/685763 for more context.
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Mojo.System.MessagesAcceptedPerEvent",
+                                static_cast<int32_t>(num_messages_accepted),
+                                1 /* min */,
+                                500 /* max */,
+                                50 /* bucket count */);
+  }
 
   AttemptShutdownIfRequested();
 }