added an opentelemetry prometheus exporter and example config to the example backend

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

packages/backend/package.json

@@ -73,6 +73,9 @@
     "@backstage/plugin-todo-backend": "workspace:^",
     "@gitbeaker/node": "^35.1.0",
     "@octokit/rest": "^19.0.3",
+    "@opentelemetry/api": "^1.4.1",
+    "@opentelemetry/exporter-prometheus": "^0.39.1",
+    "@opentelemetry/sdk-metrics": "^1.13.0",
     "azure-devops-node-api": "^11.0.1",
     "better-sqlite3": "^8.0.0",
     "dockerode": "^3.3.1",

packages/backend/prometheus.yml

@@ -0,0 +1,21 @@
+# This is an example configuration file for Prometheus, that is set to scrape
+# the metrics endpoint exposed by the example backend.
+#
+# If you want to run a docker based Prometheus instance locally to try it out,
+# run the following command in the Backstage root:
+#
+# docker run --mount type=bind,source=./packages/backend/prometheus.yml,destination=/etc/prometheus/prometheus.yml --publish published=9090,target=9090,protocol=tcp prom/prometheus
+#
+# After this, you have a Prometheus instance on http://localhost:9090
+
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  - job_name: 'prometheus'
+    static_configs:
+      # This target address specifically works on Docker for MacOS - you may
+      # need to change it to another host depending on your OS and Docker
+      # engine.
+      - targets: ['host.docker.internal:9464']

packages/backend/src/index.ts

@@ -30,7 +30,7 @@ import {
   loadBackendConfig,
   notFoundHandler,
   DatabaseManager,
-  SingleHostDiscovery,
+  HostDiscovery,
   UrlReaders,
   useHotMemoize,
   ServerTokenManager,
@@ -69,11 +69,22 @@ import { PluginEnvironment } from './types';
 import { ServerPermissionClient } from '@backstage/plugin-permission-node';
 import { DefaultIdentityClient } from '@backstage/plugin-auth-node';
 import { DefaultEventBroker } from '@backstage/plugin-events-backend';
+import { PrometheusExporter } from '@opentelemetry/exporter-prometheus';
+import { MeterProvider } from '@opentelemetry/sdk-metrics';
+import { metrics } from '@opentelemetry/api';
+
+// Expose opentelemetry metrics using a Prometheus exporter on
+// http://localhost:9464/metrics . See prometheus.yml in packages/backend for
+// more information on how to scrape it.
+const exporter = new PrometheusExporter();
+const meterProvider = new MeterProvider();
+metrics.setGlobalMeterProvider(meterProvider);
+meterProvider.addMetricReader(exporter);
 
 function makeCreateEnv(config: Config) {
   const root = getRootLogger();
   const reader = UrlReaders.default({ logger: root, config });
-  const discovery = SingleHostDiscovery.fromConfig(config);
+  const discovery = HostDiscovery.fromConfig(config);
   const tokenManager = ServerTokenManager.fromConfig(config, { logger: root });
   const permissions = ServerPermissionClient.fromConfig(config, {
     discovery,