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/gc/gcworker.py

@@ -9,8 +9,10 @@ from data.model.repository import find_repository_with_garbage, get_random_gc_po
 from data.model.gc import garbage_collect_repo
 from workers.worker import Worker
 
+
 logger = logging.getLogger(__name__)
 
+
 class GarbageCollectionWorker(Worker):
   def __init__(self):
     super(GarbageCollectionWorker, self).__init__()