Refactor metric collection

This change adds a generic queue onto which metrics can be pushed. A
separate module removes metrics from the queue and adds them to Cloudwatch.
Since these are now separate ideas, we can easily change the consumer from
Cloudwatch to anything else.

This change maintains near feature parity (the only change is there is now
just one queue instead of two - not a big deal).

data/queue.py

@@ -15,9 +15,9 @@ class NoopWith:
 
 class WorkQueue(object):
   def __init__(self, queue_name, transaction_factory,
-               canonical_name_match_list=None, reporter=None):
+               canonical_name_match_list=None, metric_queue=None):
     self._queue_name = queue_name
-    self._reporter = reporter
+    self._metric_queue = metric_queue
     self._transaction_factory = transaction_factory
     self._currently_processing = False
 
@@ -75,12 +75,14 @@ class WorkQueue(object):
     return (running_count, available_not_running_count, available_count)
 
   def update_metrics(self):
-    if self._reporter is None:
+    if self._metric_queue is None:
       return
 
     (running_count, available_not_running_count, available_count) = self.get_metrics()
-    self._reporter(self._currently_processing, running_count,
-                   running_count + available_not_running_count)
+    self._metric_queue.put('BuildCapacityShortage', available_not_running_count, unit='Count')
+
+    building_percent = 100 if self._currently_processing else 0
+    self._metric_queue.put('PercentBuilding', building_percent, unit='Percent')
 
   def has_retries_remaining(self, item_id):
     """ Returns whether the queue item with the given id has any retries remaining. If the