add spectator-js perf test script

Author: copperlight

docs/spectator/lang/nodejs/perf-test.md

@@ -5,7 +5,36 @@
 Test maximum single-threaded throughput for two minutes.
 
 ```javascript
-// tbd
+#!/usr/bin/env node
+
+import {Registry} from "nflx-spectator";
+
+// default is udp
+const registry = new Registry();
+const tags = {"location": "udp", "version": "correct-horse-battery-staple"};
+
+const max_duration = 2 * 60;
+const start = performance.now();
+
+function elapsed() {
+    return ((performance.now() - start) / 1000).toFixed(2);
+}
+
+console.log("start spectator-js udp benchmark");
+let iteration = 1;
+while (true) {
+    // without await, a heap limit allocation fail error will occur around 5.5M iterations (34 sec)
+    await registry.counter("spectator-js.publish", tags).increment();
+    if (iteration % 500000 === 0) {
+        console.log("iterations", iteration, "elapsed", elapsed());
+        if (elapsed() > max_duration) {
+            break;
+        }
+    }
+    iteration += 1;
+}
+
+console.log("iterations", iteration, "rate/sec", iteration/elapsed());
 ```
 
 ## Results