fix(@opentelemetry/exporter-prometheus): unref prometheus server to prevent process running indefinitely

Adds a test case as well, but, unfortunately, just that the server is
unrefed. Not immediately clear how we could test based on open handles
or reference counting to get at the behavior over the specific
implementation.

Author: mothershipper

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

@@ -68,7 +68,8 @@ export class PrometheusExporter implements MetricExporter {
       typeof config.appendTimestamp === 'boolean'
         ? config.appendTimestamp
         : PrometheusExporter.DEFAULT_OPTIONS.appendTimestamp;
-    this._server = createServer(this._requestHandler);
+    // unref to prevent prometheus exporter from holding the process open on exit
+    this._server = createServer(this._requestHandler).unref();
     this._serializer = new PrometheusSerializer(
       this._prefix,
       this._appendTimestamp

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

@@ -36,6 +36,7 @@ describe('PrometheusExporter', () => {
   mockAggregator(HistogramAggregator);
 
   afterEach(() => {
+    sinon.restore();
     delete process.env.OTEL_EXPORTER_PROMETHEUS_HOST;
     delete process.env.OTEL_EXPORTER_PROMETHEUS_PORT;
   });
@@ -116,6 +117,16 @@ describe('PrometheusExporter', () => {
       );
     });
 
+    it('should unref the server to allow graceful termination', () => {
+      const mockServer = sinon.createStubInstance(http.Server);
+      const createStub = sinon.stub(http, 'createServer');
+      createStub.returns((mockServer as any) as http.Server);
+      const exporter = new PrometheusExporter({}, async () => {
+        await exporter.shutdown();
+      });
+      sinon.assert.calledOnce(mockServer.unref);
+    });
+
     it('should listen on environmentally set host and port', () => {
       process.env.OTEL_EXPORTER_PROMETHEUS_HOST = '127.0.0.1';
       process.env.OTEL_EXPORTER_PROMETHEUS_PORT = '1234';