Adds a simple indicator to display FPS (#3624)

* Adds a simple indicator to display FPS

* ratchet up test coverage threshold

src/plugins/performanceIndicator/plugin.js

@@ -0,0 +1,62 @@
+/*****************************************************************************
+ * Open MCT, Copyright (c) 2014-2020, United States Government
+ * as represented by the Administrator of the National Aeronautics and Space
+ * Administration. All rights reserved.
+ *
+ * Open MCT is licensed under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ * Open MCT includes source code licensed under additional open source
+ * licenses. See the Open Source Licenses file (LICENSES.md) included with
+ * this source code distribution or the Licensing information page available
+ * at runtime from the About dialog for additional information.
+ *****************************************************************************/
+export default function PerformanceIndicator() {
+    return function install(openmct) {
+        let frames = 0;
+        let lastCalculated = performance.now();
+        const indicator = openmct.indicators.simpleIndicator();
+
+        indicator.text('~ fps');
+        indicator.statusClass('s-status-info');
+        openmct.indicators.add(indicator);
+
+        let rafHandle = requestAnimationFrame(incremementFrames);
+
+        openmct.on('destroy', () => {
+            cancelAnimationFrame(rafHandle);
+        });
+
+        function incremementFrames() {
+            let now = performance.now();
+            if ((now - lastCalculated) < 1000) {
+                frames++;
+            } else {
+                updateFPS(frames);
+                lastCalculated = now;
+                frames = 1;
+            }
+
+            rafHandle = requestAnimationFrame(incremementFrames);
+        }
+
+        function updateFPS(fps) {
+            indicator.text(`${fps} fps`);
+            if (fps >= 40) {
+                indicator.statusClass('s-status-on');
+            } else if (fps < 40 && fps >= 20) {
+                indicator.statusClass('s-status-warning');
+            } else {
+                indicator.statusClass('s-status-error');
+            }
+        }
+    };
+}