Only install monotonic on python2

monotonic is a backport of time.monotonic from python3. Only install it
for python2.

Depends-On: https://review.openstack.org/615441
Change-Id: Id27cd748e883d54dd93dac2e6bd8caee6728f7e1

ceilometer/compute/pollsters/__init__.py

@@ -13,8 +13,13 @@
 # under the License.
 
 import collections
+import six
+
+if six.PY2:
+    from monotonic import monotonic as now
+else:
+    from time import monotonic as now
 
-import monotonic
 from oslo_log import log
 from oslo_utils import timeutils
 
@@ -94,7 +99,7 @@ class GenericComputePollster(plugin_base.PollsterBase):
         if instance.id not in cache[self.inspector_method]:
             result = getattr(self.inspector, self.inspector_method)(
                 instance, duration)
-            polled_time = monotonic.monotonic()
+            polled_time = now()
             # Ensure we don't cache an iterator
             if isinstance(result, collections.Iterable):
                 result = list(result)

requirements.txt

@@ -8,7 +8,7 @@ futures>=3.0;python_version=='2.7' or python_version=='2.6' # BSD
 futurist>=1.8.0 # Apache-2.0
 jsonpath-rw-ext>=1.1.3 # Apache-2.0
 lxml>=3.4.1 # BSD
-monotonic>=0.6
+monotonic>=0.6;python_version<'3.3' # Apache-2.0
 msgpack>=0.5.0 # Apache-2.0
 oslo.concurrency>=3.26.0 # Apache-2.0
 oslo.config>=5.2.0 # Apache-2.0