Avoid logging very small values for Mojo.System.MessagesAcceptedPerEvent

mojo/edk/system/node_controller.cc

 // Copyright 2016 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.
 
 #include "mojo/edk/system/node_controller.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/bind.h"
@@ skipping 737 matching lines @@
       node_->AcceptMessage(std::move(messages.front()));
       messages.pop();
     }
 
     // This is effectively a safeguard against potential bugs which might lead
     // to runaway message cycles. If any such cycles arise, we'll start seeing
     // crash reports from this location.
     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();
 }
 
 void NodeController::ProcessIncomingMessages() {
   RequestContext request_context(RequestContext::Source::SYSTEM);
 
   {
     base::AutoLock lock(messages_lock_);
     // Allow a new incoming messages processing task to be posted. This can't be
@@ skipping 671 matching lines @@
 NodeController::PeerConnection::~PeerConnection() = default;
 
 NodeController::PeerConnection& NodeController::PeerConnection::
 operator=(const PeerConnection& other) = default;
 
 NodeController::PeerConnection& NodeController::PeerConnection::
 operator=(PeerConnection&& other) = default;
 
 }  // namespace edk
 }  // namespace mojo