net: Add some metrics on reuse of idle sockets.

We're investigating keeping fewer of them around, and/or timing them out
more aggressively to decrease memory usage.

BUG=621197

Review-Url: https://codereview.chromium.org/2086903003
Cr-Commit-Position: refs/heads/master@{#401370}

net/socket/client_socket_pool_base.cc

@@ -11,6 +11,7 @@
 #include "base/format_macros.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
 #include "base/strings/string_util.h"
@@ -975,11 +976,21 @@ void ClientSocketPoolBaseHelper::HandOutSocket(
   handle->set_pool_id(pool_generation_number_);
   handle->set_connect_timing(connect_timing);
 
-  if (handle->is_reused()) {
+  if (reuse_type == ClientSocketHandle::REUSED_IDLE) {
     net_log.AddEvent(
         NetLog::TYPE_SOCKET_POOL_REUSED_AN_EXISTING_SOCKET,
         NetLog::IntCallback("idle_ms",
                             static_cast<int>(idle_time.InMilliseconds())));
+
+    UMA_HISTOGRAM_COUNTS_1000("Net.Socket.IdleSocketReuseTime",
+                              idle_time.InSeconds());
+  }
+
+  if (reuse_type != ClientSocketHandle::UNUSED) {
+    // The socket being handed out is no longer considered idle, but was
+    // considered idle until just before this method was called.
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Net.Socket.NumIdleSockets",
+                                idle_socket_count() + 1, 1, 256, 50);
   }
 
   net_log.AddEvent(

tools/metrics/histograms/histograms.xml

@@ -30087,6 +30087,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.Socket.IdleSocketReuseTime" units="seconds">
+  <owner>mmenke@chromium.org</owner>
+  <summary>
+    Seconds a socket was idle before it was reused. Emitted upon reuse. Does not
+    record the times sockets were idle before first use.
+  </summary>
+</histogram>
+
+<histogram name="Net.Socket.NumIdleSockets">
+  <owner>mmenke@chromium.org</owner>
+  <summary>Number of idle sockets when one of them was reused.</summary>
+</histogram>
+
 <histogram name="Net.SocketIdleTimeBeforeNextUse_ReusedSocket">
   <obsolete>
     Deprecated as of 03/2015.