feat(prometheus): add getMetricsRequestHandler-method to Prometheus…

… exporter

This commit introduces a new method on the `PrometheusExporter`-class that
allows users of server frameworks with an existing service instance to
call the `getMetricsRequestHandler`-method as part of a route to return
the collected metrics in Prometheus format.

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

@@ -157,6 +157,18 @@ export class PrometheusExporter implements MetricExporter {
     });
   }
 
+  /**
+   * Request handler that responds with the current state of metrics
+   * @param request Incoming HTTP request of server instance
+   * @param response HTTP response objet used to response to request
+   */
+  public getMetricsRequestHandler(
+    _request: IncomingMessage,
+    response: ServerResponse
+  ) {
+    this._exportMetrics(response);
+  }
+
   /**
    * Request handler used by http library to respond to incoming requests
    * for the current state of metrics by the Prometheus backend.

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

@@ -171,6 +171,27 @@ describe('PrometheusExporter', () => {
         return done();
       });
     });
+
+    it('should able to call getMetricsRequestHandler function to generate response with metrics', () => {
+      const exporter = new PrometheusExporter({ preventServerStart: true });
+      const mockRequest = {} as http.IncomingMessage;
+      let calledEnd = false;
+      let calledSetHeader = false;
+      const mockResponse = {
+        statusCode: 0,
+        setHeader: (_name: string, _value: string) => {
+          calledSetHeader = true;
+        },
+        end: () => {
+          calledEnd = true;
+        },
+      } as http.ServerResponse;
+
+      exporter.getMetricsRequestHandler(mockRequest, mockResponse);
+      assert.strictEqual(calledEnd, true);
+      assert.strictEqual(calledSetHeader, true);
+      assert.strictEqual(mockResponse.statusCode, 200);
+    });
   });
 
   describe('export', () => {