Add simple malloc statistics and metrics

Add some basic statistics for allocated memory based on mallinfo(),
and use that to define two metrics:

 usedAfterOverview: bytes used after the overview is shown once
 leakedAfterOverview: additional bytes used when the overview is
   shown a second time.

https://bugzilla.gnome.org/show_bug.cgi?id=618189

src/gnome-shell-plugin.c

@@ -38,12 +38,14 @@
 #include <gjs/gjs.h>
 #include <girepository.h>
 #include <gmodule.h>
+#include <malloc.h>
 #include <stdlib.h>
 #include <string.h>
 
 #include "display.h"
 
 #include "shell-global-private.h"
+#include "shell-perf-log.h"
 #include "shell-wm.h"
 #include "st.h"
 
@@ -232,6 +234,50 @@ settings_notify_cb (GtkSettings *settings,
   update_font_options (settings);
 }
 
+static void
+malloc_statistics_callback (ShellPerfLog *perf_log,
+                            gpointer      data)
+{
+  struct mallinfo info = mallinfo ();
+
+  shell_perf_log_update_statistic_i (perf_log,
+                                     "malloc.arenaSize",
+                                     info.arena);
+  shell_perf_log_update_statistic_i (perf_log,
+                                     "malloc.mmapSize",
+                                     info.hblkhd);
+  shell_perf_log_update_statistic_i (perf_log,
+                                     "malloc.usedSize",
+                                     info.uordblks);
+}
+
+static void
+add_statistics (GnomeShellPlugin *shell_plugin)
+{
+  ShellPerfLog *perf_log = shell_perf_log_get_default ();
+
+  /* For probably historical reasons, mallinfo() defines the returned values,
+   * even those in bytes as int, not size_t. We're determined not to use
+   * more than 2G of malloc'ed memory, so are OK with that.
+   */
+  shell_perf_log_define_statistic (perf_log,
+                                   "malloc.arenaSize",
+                                   "Amount of memory allocated by malloc() with brk(), in bytes",
+                                   "i");
+  shell_perf_log_define_statistic (perf_log,
+                                   "malloc.mmapSize",
+                                   "Amount of memory allocated by malloc() with mmap(), in bytes",
+                                   "i");
+  shell_perf_log_define_statistic (perf_log,
+                                   "malloc.usedSize",
+                                   "Amount of malloc'ed memory currently in use",
+                                   "i");
+
+  shell_perf_log_add_statistics_callback (perf_log,
+                                          malloc_statistics_callback,
+                                          NULL, NULL);
+}
+
 static void
 gnome_shell_plugin_start (MutterPlugin *plugin)
 {
@@ -280,6 +326,8 @@ gnome_shell_plugin_start (MutterPlugin *plugin)
   _shell_global_set_plugin (global, MUTTER_PLUGIN(shell_plugin));
   _shell_global_set_gjs_context (global, shell_plugin->gjs_context);
 
+  add_statistics (shell_plugin);
+
   if (!gjs_context_eval (shell_plugin->gjs_context,
                          "const Main = imports.ui.main; Main.start();",
                          -1,