Add process CPU usage metrics

app.js

@@ -128,6 +128,40 @@ listener.on('listening', function() {
     );
 });
 
+function getCPUUsage (oldUsage) {
+    let usage;
+
+    if (oldUsage && oldUsage._start) {
+        usage = Object.assign({}, process.cpuUsage(oldUsage._start.cpuUsage));
+        usage.time = Date.now() - oldUsage._start.time;
+    } else {
+        usage = Object.assign({}, process.cpuUsage());
+        usage.time = process.uptime() * 1000; // s to ms
+    }
+
+    usage.percent = (usage.system + usage.user) / (usage.time * 10);
+
+    Object.defineProperty(usage, '_start', {
+        value: {
+            cpuUsage: process.cpuUsage(),
+            time: Date.now()
+        }
+    });
+
+    return usage;
+}
+
+let previousCPUUsage = getCPUUsage();
+setInterval(function cpuUsageMetrics () {
+    const CPUUsage = getCPUUsage(previousCPUUsage);
+
+    Object.keys(CPUUsage).forEach(property => {
+        global.statsClient.gauge(`windshaft.cpu.${property}`, CPUUsage[property]);
+    });
+
+    previousCPUUsage = CPUUsage;
+}, 5000);
+
 setInterval(function() {
     var memoryUsage = process.memoryUsage();
     Object.keys(memoryUsage).forEach(function(k) {