Move the metrics update to get_metrics

It is not necessary to update the metrics when get every metric in
one compute monitor plugin, just update once when get metrics for
the monitor plugin.

Change-Id: Ib665188964de8318f6d94343891666c674111c8c

nova/compute/monitors/__init__.py

@@ -83,28 +83,20 @@ class ResourceMonitorBase(object):
         which means a timestamp should be added into the returned value.
         That is, a tuple (value, timestamp) is returned.
 
-        The timestamp is not the time when the function is called but probably
-        when the value the function returns was retrieved.
-        Actually the value is retrieved by the internal method
-        _update_data(). Because we don't allow _update_data() is called
-        so frequently. So, the value is read from the cache which was got in
-        the last call sometimes.
+        The timestamp is the time when we update the value in the _data.
 
-        If users want to use this decorator, they need to implement class
-        method _update_data() and variable _data.
         If users hope to define how the timestamp is got by themselves,
         they should not use this decorator in their own classes.
         """
         def wrapper(cls, **kwargs):
-            cls._update_data()
             return func(cls, **kwargs), cls._data.get("timestamp", None)
         return wrapper
 
     def _update_data(self):
         """Method to update the metrics data.
 
-        Each subclass should implement this method to update metrics.
-        It will be called in the decorator add_timestamp.
+        Each subclass can implement this method to update metrics
+        into _data. It will be called in get_metrics.
         """
         pass
 
@@ -129,6 +121,7 @@ class ResourceMonitorBase(object):
         :returns: a list to tell the current metrics
         """
         data = []
+        self._update_data()
         for name, func in self.metric_map.iteritems():
             ret = func(self, **kwargs)
             data.append(self._populate(name, ret[0], ret[1]))

nova/tests/compute/monitors/test_monitors.py

@@ -20,15 +20,18 @@ from nova import test
 
 
 class FakeResourceMonitor(monitors.ResourceMonitorBase):
-    def get_metric_names(self):
-        return ["foo.metric1", "foo.metric2"]
+    def _update_data(self):
+        self._data['foo.metric1'] = '1000'
+        self._data['foo.metric2'] = '99.999'
+        self._data['timestamp'] = '123'
 
-    def get_metrics(self):
-        data = []
-        data.append(self._populate('foo.metric1', '1000'))
-        data.append(self._populate('foo.metric2', '99.999'))
+    @monitors.ResourceMonitorBase.add_timestamp
+    def _get_foo_metric1(self, **kwargs):
+        return self._data.get("foo.metric1")
 
-        return data
+    @monitors.ResourceMonitorBase.add_timestamp
+    def _get_foo_metric2(self, **kwargs):
+        return self._data.get("foo.metric2")
 
 
 class FakeMonitorClass1(monitors.ResourceMonitorBase):
@@ -96,6 +99,7 @@ class ResourceMonitorBaseTestCase(test.TestCase):
         metrics = {}
         for metric in metrics_raw:
             self.assertIn(metric['name'], names)
+            self.assertEqual(metric["timestamp"], '123')
             metrics[metric['name']] = metric['value']
 
         self.assertEqual(metrics["foo.metric1"], '1000')