refactor(exporter-prometheus): promisify prometheus tests

Author: legendecas

doc/metrics.md

@@ -59,7 +59,7 @@ const {
   getNodeAutoInstrumentations,
 } = require("@opentelemetry/auto-instrumentations-node");
 
-const prometheusExporter = new PrometheusExporter({ startServer: true });
+const prometheusExporter = new PrometheusExporter();
 
 const sdk = new opentelemetry.NodeSDK({
   // Optional - If omitted, the metrics SDK will not be initialized
@@ -147,7 +147,7 @@ const {
   getNodeAutoInstrumentations,
 } = require("@opentelemetry/auto-instrumentations-node");
 
-const prometheusExporter = new PrometheusExporter({ startServer: true });
+const prometheusExporter = new PrometheusExporter();
 
 const sdk = new opentelemetry.NodeSDK({
   // Optional - If omitted, the metrics SDK will not be initialized
@@ -499,8 +499,8 @@ to use the Prometheus exporter `PrometheusExporter` which is included in the
 const { PrometheusExporter } = require('@opentelemetry/exporter-prometheus');
 const { MeterProvider }  = require('@opentelemetry/sdk-metrics');
 
-// Add your port and startServer to the Prometheus options
-const options = { port: 9464, startServer: true };
+// Add your port to the Prometheus options
+const options = { port: 9464 };
 const exporter = new PrometheusExporter(options);
 
 // Creates MeterProvider and installs the exporter as a MetricReader

experimental/packages/opentelemetry-exporter-prometheus/src/PrometheusExporter.ts

@@ -44,6 +44,7 @@ export class PrometheusExporter extends MetricReader {
   private readonly _prefix?: string;
   private readonly _appendTimestamp: boolean;
   private _serializer: PrometheusSerializer;
+  private _startServerPromise: Promise<void> | undefined;
 
   // This will be required when histogram is implemented. Leaving here so it is not forgotten
   // Histogram cannot have a attribute named 'le'
@@ -95,7 +96,8 @@ export class PrometheusExporter extends MetricReader {
         callback(err);
       });
     } else if (callback) {
-      callback();
+      // Do not invoke callback immediately to avoid zalgo problem.
+      queueMicrotask(callback);
     }
   }
 
@@ -142,7 +144,7 @@ export class PrometheusExporter extends MetricReader {
    * Starts the Prometheus export server
    */
   startServer(): Promise<void> {
-    return new Promise((resolve, reject) => {
+    this._startServerPromise ??= new Promise((resolve, reject) => {
       this._server.once('error', reject);
       this._server.listen(
         {
@@ -157,6 +159,8 @@ export class PrometheusExporter extends MetricReader {
         }
       );
     });
+
+    return this._startServerPromise;
   }
 
   /**