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/api/templates/api/metrics_view.md

@@ -0,0 +1 @@
+query params to filter response, e.g., ?subsystemonly=1&metric=callback_receiver_events_insert_db&node=awx-1