Add UtilizationMonitor: CPU, memory and disk usage from /proc

meson.build

@@ -234,6 +234,7 @@ if get_option('system')
     'src/system/autostart_manager.vala',
     'src/system/hardware_info.vala',
     'src/system/sensor.vala',
+    'src/system/utilization.vala',
   )
 
   libsingularity_system = shared_library('singularity-system',
@@ -289,6 +290,23 @@ if get_option('system')
     dependencies: [gio_dep, gee_dep],
   )
   test('sensor', sensor_test)
+
+  utilization_test = executable(
+    'utilization-test',
+    files(
+      'tests/utilization_test.vala',
+      'src/system/utilization.vala',
+      'src/system/sensor.vala',
+    ),
+    # giounix_dep: utilization.vala's is_real_filesystem() consults
+    # GLib.UnixMountEntry (gio-unix-2.0) when proc_root is unset. The
+    # fixture tests always set proc_root, so this path never actually runs
+    # under test -- but the file still needs to LINK, and every valac
+    # invocation compiles the whole file regardless of which branch a test
+    # exercises.
+    dependencies: [gio_dep, giounix_dep, gee_dep],
+  )
+  test('utilization', utilization_test)
 endif
 
 # Build the Singularity GTK3/GTK4 theme for third-party apps. Orchis supplies