Fix wrong CPU metric value in metrics_filter

CPU metrics are reported as a normalized value in [0,1].
When scheduler reads the data as MonitorMetric object, as part
of update_from_compute_node(), the value of the metric gets
lost due to a wrong type conversion, typecasting into an Integer,
reduces it to 0.

Note:
 - It may seem unintuitive that to fix the same, we are not changing
   the type of metric from Integer to Float. Please refer the
   discussions -
   * https://review.openstack.org/#/c/243825/
   * https://review.openstack.org/#/c/216923/
 - To summarize, we are trying to save some 'possible' downstream consumer
   of metrics updates from compute nodes, i.e. consuming from AMPQ, from
   software upgrade scenarios.

Change-Id: Ib504af33e05dfc4d7e97b52682e27befc67d784a
Closes-bug: #1514997

nova/objects/monitor_metric.py

@@ -93,8 +93,18 @@ def from_json(cls, metrics):
         :returns: a MonitorMetricList Object.
         """
         metrics = jsonutils.loads(metrics) if metrics else []
-        metric_list = [
-            MonitorMetric(**metric) for metric in metrics]
+
+        # NOTE(suro-patz): While instantiating the MonitorMetric() from
+        #                  JSON-ified string, we need to re-convert the
+        #                  normalized metrics to avoid truncation to 0 by
+        #                  typecasting into an integer.
+        metric_list = []
+        for metric in metrics:
+            if ('value' in metric and metric['name'] in
+                                      FIELDS_REQUIRING_CONVERSION):
+                metric['value'] = metric['value'] * 100
+            metric_list.append(MonitorMetric(**metric))
+
         return MonitorMetricList(objects=metric_list)
 
     # NOTE(jaypipes): This method exists to convert the object to the

nova/tests/unit/objects/test_monitor_metric.py

@@ -10,6 +10,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from oslo_serialization import jsonutils
 from oslo_utils import timeutils
 
 from nova import objects
@@ -73,6 +74,17 @@ def test_monitor_NUMA_metric_to_dict(self):
                                     source='nova.virt.libvirt.driver')
         self.assertEqual(_monitor_numa_metric_spec, obj.to_dict())
 
+    def test_conversion_in_monitor_metric_list_from_json(self):
+        spec_list = [_monitor_metric_spec, _monitor_metric_perc_spec]
+        metrics = objects.MonitorMetricList.from_json(
+            jsonutils.dumps(spec_list))
+        for metric, spec in zip(metrics, spec_list):
+            exp = spec['value']
+            if (spec['name'] in
+                    objects.monitor_metric.FIELDS_REQUIRING_CONVERSION):
+                exp = spec['value'] * 100
+            self.assertEqual(exp, metric.value)
+
 
 class TestMonitorMetricObject(test_objects._LocalTest,
                               _TestMonitorMetricObject):