util/metrics: remove metricqueue abstraction

This change replaces the metricqueue library with a native Prometheus
client implementation with the intention to aggregated results with the
Prometheus PushGateway.

This change also adds instrumentation for greenlet context switches.

workers/notificationworker/test/test_notificationworker.py

@@ -14,6 +14,7 @@ from test.fixtures import *
 
 from workers.notificationworker.models_pre_oci import pre_oci_model as model
 
+
 def test_basic_notification_endtoend(initialized_db):
   # Ensure the public user doesn't have any notifications.
   assert not model.user_has_local_notifications('public')