Skip a cycle instead of retrying if a metric is not found in gnocchi

In case a metric is not found in gnocchi, return nothing instead of retrying.
The metric is unlikely to be available during the next collection cycle.

Change-Id: I4b954b146b800255554449bb19eb4bed3cb87dd4

cloudkitty/collector/gnocchi.py

@@ -13,8 +13,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 #
+import six
+
 from gnocchiclient import auth as gauth
 from gnocchiclient import client as gclient
+from gnocchiclient import exceptions as gexceptions
 from keystoneauth1 import loading as ks_loading
 from oslo_config import cfg
 from oslo_log import log as logging
@@ -309,13 +312,23 @@ def _fetch_metric(self, metric_name, start, end,
         # get groupby
         groupby = self.conf[metric_name]['groupby']
 
-        return self._conn.aggregates.fetch(
-            op,
-            resource_type=resource_type,
-            start=ck_utils.ts2dt(start),
-            stop=ck_utils.ts2dt(end),
-            groupby=groupby,
-            search=self.extend_filter(*query_parameters))
+        try:
+            return self._conn.aggregates.fetch(
+                op,
+                resource_type=resource_type,
+                start=ck_utils.ts2dt(start),
+                stop=ck_utils.ts2dt(end),
+                groupby=groupby,
+                search=self.extend_filter(*query_parameters))
+        except (gexceptions.MetricNotFound, gexceptions.BadRequest) as e:
+            # FIXME(peschk_l): gnocchiclient seems to be raising a BadRequest
+            # when it should be raising MetricNotFound
+            if isinstance(e, gexceptions.BadRequest):
+                if 'Metrics not found' not in six.text_type(e):
+                    raise
+            LOG.warning('[{scope}] Skipping this metric for the '
+                        'current cycle.'.format(scope=project_id, err=e))
+            return []
 
     def _format_data(self, metconf, data, resources_info=None):
         """Formats gnocchi data to CK data.

releasenotes/notes/add-scope-key-58135c2a5c6dae68.yaml

@@ -1,5 +1,5 @@
 ---
 other:
   - |
-    The "scope_key" option is now defained in cloudkitty.conf and has been
+    The "scope_key" option is now defined in cloudkitty.conf and has been
     removed from the cloudkitty and monasca collector's extra_args

releasenotes/notes/skip-period-if-nonexistent-metric-ba56a671e68f5bf5.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The behaviour of the gnocchi collector has changed in case of a nonexistent
+    metric. Given that a nonexistent metric is unlikely to exist in the next
+    collection cycle, the metric is simply skipped.