Improve metric error handling

flush() should not produce a metric if value is None (causes
exception in monasca-api).
Fix the exception message in flush() to use the correct data
structure.

Change-Id: I62d31270db9e70f16d2a38a73856009c52c098e6
Story: 2004276
Task: 27825

monasca_agent/common/aggregator.py

@@ -54,7 +54,7 @@ class MetricsAggregator(object):
             try:
                 metrics.extend(metric.flush())
             except Exception:
-                log.exception('Error flushing {0} metrics.'.format(metric.name))
+                log.exception('Error flushing {0} metrics.'.format(metric.metric['name']))
 
         # Log a warning regarding metrics with old timestamps being submitted
         if self.num_discarded_old_points > 0:

monasca_agent/common/metrics.py

@@ -53,6 +53,13 @@ class Metric(object):
     def flush(self):
         if self.timestamp is None:
             return []
+        if self.value is None:
+            log.error('Value of None is not supported, will not send. '
+                      'Metric %s with dimensions %s at time %d',
+                      self.metric['name'],
+                      self.metric['dimensions'],
+                      self.timestamp)
+            return []
 
         envelope = self.measurement(self.value, self.timestamp)
         self.timestamp = None