Add subsystem metrics

- Adds a Metrics() class that can track data such as number of
events the callback receiver inserted into database
- Exposes this metric data at the api/v2/metrics/ endpoint.
This data is prometheus-friendly
- Metric data is stored in memory, then periodically saved to Redis.
- Metric data is periodically broadcast to other nodes in the cluster,
so that each node has a copy of the most recent metric data collected.

awx/main/consumers.py

@@ -13,7 +13,6 @@ from channels.generic.websocket import AsyncJsonWebsocketConsumer
 from channels.layers import get_channel_layer
 from channels.db import database_sync_to_async
 
-
 logger = logging.getLogger('awx.main.consumers')
 XRF_KEY = '_auth_user_xrf'