An abstract class for capturing nomad stats

It follows the form of poll -> json parse -> append,
Where append is defined in subclasses to add data from the new frame
to long-lived rolling arrays of data.

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

@@ -0,0 +1,27 @@
+import Mixin from '@ember/object/mixin';
+import { assert } from '@ember/debug';
+
+export default Mixin.create({
+  url: '',
+
+  fetch() {
+    assert('StatTrackers need a fetch method, which should have an interface like window.fetch');
+  },
+
+  append(/* frame */) {
+    assert(
+      'StatTrackers need an append method, which takes the JSON response from a request to url as an argument'
+    );
+  },
+
+  poll() {
+    const url = this.get('url');
+    assert('Url must be defined', url);
+
+    return this.get('fetch')(url)
+      .then(res => {
+        return res.json();
+      })
+      .then(frame => this.append(frame));
+  },
+});