Use prometheus as a metric backend

This entails writing a metric aggregation program since each worker has its
own memory, and thus own metrics because of python gunicorn. The python
client is a simple wrapper that makes web requests to it.

app.py

@@ -25,6 +25,7 @@ from data.archivedlogs import LogArchive
 from data.userevent import UserEventsBuilderModule
 from data.queue import WorkQueue, MetricQueueReporter
 from util import get_app_url
+from util import prometheus
 from util.saas.analytics import Analytics
 from util.saas.exceptionlog import Sentry
 from util.names import urn_generator
@@ -207,6 +208,8 @@ if os.path.exists(_v2_key_path):
 else:
   docker_v2_signing_key = RSAKey(key=RSA.generate(2048))
 
+prometheus.init(app.config.get('PROMETHEUS_AGGREGATOR_URL'))
+
 database.configure(app.config)
 model.config.app_config = app.config
 model.config.store = storage