Add support for limiting the number of samples returned

Blueprint: api-limit

Change-Id: Id053eb60674fea58b3d83b460fd0344dbc050cbf
Signed-off-by: Julien Danjou <julien@danjou.info>

ceilometer/storage/impl_mongodb.py

@@ -440,12 +440,16 @@ class Connection(base.Connection):
                     user_id=r['user_id'],
                 )
 
-    def get_samples(self, sample_filter):
-        """Return an iterable of samples as created by
-        :func:`ceilometer.meter.meter_message_from_counter`.
+    def get_samples(self, sample_filter, limit=None):
+        """Return an iterable of model.Sample instances.
+
+        :param sample_filter: Filter.
+        :param limit: Maximum number of results to return.
         """
+        if limit == 0:
+            return
         q = make_query_from_filter(sample_filter, require_meter=False)
-        samples = self.db.meter.find(q)
+        samples = self.db.meter.find(q).limit(limit or 0)
         for s in samples:
             # Remove the ObjectId generated by the database when
             # the sample was inserted. It is an implementation