Do not log a warning about not using compute monitors

The compute_monitors configuration option is empty by
default, however, on nova-compute startup we get a warning
logged saying:

  Excluding nova.compute.monitors.cpu monitor virt_driver. \
  Not in the list of enabled monitors (CONF.compute_monitors).

This is unnecessary noise in the logs when nova-compute is
not configured to use monitors, so this change avoids the warning
log message in that case.

The compute_monitors configuration option help text is also
updated to (1) fix formatting and (2) remove the mention of the
numa_mem_bw monitor which never made it into nova (see
blueprint memory-bw).

Change-Id: I5bd204e0017f6f795e3cf13b34b75124ee23aa78
Closes-Bug: #1823207

nova/compute/monitors/__init__.py

@@ -84,8 +84,10 @@ class MonitorHandler(object):
         if namespace + '.' + ext.name in cfg_monitors:
             self.type_monitor_loaded[namespace] = ext.name
             return True
-        LOG.warning("Excluding %(namespace)s monitor %(monitor_name)s. "
-                    "Not in the list of enabled monitors "
-                    "(CONF.compute_monitors).",
-                    {'namespace': namespace, 'monitor_name': ext.name})
+        # Only log something if compute_monitors is not empty.
+        if CONF.compute_monitors:
+            LOG.warning("Excluding %(namespace)s monitor %(monitor_name)s. "
+                        "Not in the list of enabled monitors "
+                        "(CONF.compute_monitors).",
+                        {'namespace': namespace, 'monitor_name': ext.name})
         return False

nova/conf/compute.py

@@ -113,10 +113,10 @@ a time.
 Possible values:
 
 * An empty list will disable the feature (Default).
-* An example value that would enable both the CPU and NUMA memory
-  bandwidth monitors that use the virt driver variant:
+* An example value that would enable the CPU
+  bandwidth monitor that uses the virt driver variant::
 
-    compute_monitors = cpu.virt_driver, numa_mem_bw.virt_driver
+    compute_monitors = cpu.virt_driver
 """),
     cfg.StrOpt('default_ephemeral_format',
         help="""

nova/tests/unit/compute/monitors/test_monitors.py

@@ -51,3 +51,12 @@ class MonitorsTestCase(test.NoDBTestCase):
                                'mon2')
         self.assertTrue(handler.check_enabled_monitor(ext_cpu_mon1))
         self.assertFalse(handler.check_enabled_monitor(ext_cpu_mon2))
+
+        # Run the check but with no monitors enabled to make sure we don't log.
+        self.flags(compute_monitors=[])
+        handler = monitors.MonitorHandler(None)
+        ext_cpu_mon1 = FakeExt('nova.compute.monitors.cpu.virt_driver:Monitor',
+                               'mon1')
+        with mock.patch.object(monitors.LOG, 'warning') as mock_warning:
+            self.assertFalse(handler.check_enabled_monitor(ext_cpu_mon1))
+        mock_warning.assert_not_called()