Add subsystem metrics for the dispatcher (#13989)

This adds a handful of metrics to /api/v2/metrics/ recorded from the dispatcher main process

Adds logic in the dispatcher period tasks to calculate these for the last collection interval
Reports worker count, task count, scale up events, and availability

Add data to demo grafana dashboard

awx/main/analytics/subsystem_metrics.py

@@ -209,6 +209,11 @@ class Metrics:
             SetFloatM('workflow_manager_recorded_timestamp', 'Unix timestamp when metrics were last recorded'),
             SetFloatM('workflow_manager_spawn_workflow_graph_jobs_seconds', 'Time spent spawning workflow tasks'),
             SetFloatM('workflow_manager_get_tasks_seconds', 'Time spent loading workflow tasks from db'),
+            # dispatcher subsystem metrics
+            SetIntM('dispatcher_pool_scale_up_events', 'Number of times local dispatcher scaled up a worker since startup'),
+            SetIntM('dispatcher_pool_active_task_count', 'Number of active tasks in the worker pool when last task was submitted'),
+            SetIntM('dispatcher_pool_max_worker_count', 'Highest number of workers in worker pool in last collection interval, about 20s'),
+            SetFloatM('dispatcher_availability', 'Fraction of time (in last collection interval) dispatcher was able to receive messages'),
         ]
         # turn metric list into dictionary with the metric name as a key
         self.METRICS = {}