Add request throttling to the abstract stats tracker

This is the best of three options

1. Users of stats trackers control polling (old method)
2. Stat tracker is stateful and has start/stop methods (like logging)
3. Stat trackers blindly throttle requests

This is the best option because it means N number of concurrent users of
a stats tracker can request polling without inundating the tracker with
redundant frames (or the network with redundant requests), but they also
don't have to coordinate amongst themselves to determine what state a
tracker should be in.

ui/app/components/primary-metric.js

@@ -79,8 +79,8 @@ export default Component.extend({
 
   poller: task(function*() {
     do {
-      yield this.get('tracker').poll();
-      yield timeout(2000);
+      yield this.get('tracker.poll').perform();
+      yield timeout(10);
     } while (!Ember.testing);
   }),
 

ui/app/utils/classes/abstract-stats-tracker.js

@@ -1,5 +1,6 @@
 import Mixin from '@ember/object/mixin';
 import { assert } from '@ember/debug';
+import { task, timeout } from 'ember-concurrency';
 
 export default Mixin.create({
   url: '',
@@ -16,12 +17,18 @@ export default Mixin.create({
     );
   },
 
-  poll() {
+  // Uses EC as a form of debounce to prevent multiple
+  // references to the same tracker from flooding the tracker,
+  // but also avoiding the issue where different places where the
+  // same tracker is used needs to coordinate.
+  poll: task(function*() {
     const url = this.get('url');
     assert('Url must be defined', url);
 
-    return this.get('fetch')(url)
+    yield this.get('fetch')(url)
       .then(res => res.json())
       .then(frame => this.append(frame));
-  },
+
+    yield timeout(2000);
+  }).drop(),
 });