feat: Add prometheus middleware to express

This enables collecting metrics from node.js and express.

src/app.ts

@@ -26,6 +26,7 @@ import {Express} from 'express';
 import morgan from 'morgan';
 import {join} from 'path';
 import {configFile, DEFAULT_TIMEOUT, isTestEnvironment, mailer, plugins, validator} from './common';
+import {getPrometheusMiddleware} from './middleware/prometheus';
 import {MailQueue} from './notification/mail-queue';
 import {bulkAddRouter} from './routes/bulk-add-route';
 import {bulkDoneRouter} from './routes/bulk-done-route';
@@ -63,6 +64,10 @@ export async function configureApp(app: Express, databases: {[name: string]: Dat
     }, stream: process.stdout,
   }));
 
+  if (process.env.PROMETHEUS_MIDDLEWARE === 'true') {
+    app.use(getPrometheusMiddleware());
+  }
+
   const corsOptions = {
     allowedHeaders: [
       'DNT',

src/middleware/prometheus.ts

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2021 StApps
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+import {Logger} from '@openstapps/logger';
+import express from 'express';
+import expressPrometheusMiddleware from 'express-prometheus-middleware';
+import fs from 'fs';
+import path from 'path';
+
+type UserOptions = Parameters<typeof expressPrometheusMiddleware>[0];
+
+/**
+ * Create and configure a new Express Prometheus Middleware instance
+ *
+ * This function tries to configure the new instance with JSON read from
+ * `./conf/prometheus.json`. When this fails an instance configured with 
+ * default options is returned.
+ * 
+ * @returns express.Express
+ */
+export function getPrometheusMiddleware(): express.Express {
+  const configFileName = path.join('./config', 'prometheus.json');
+  let options: UserOptions = {};
+
+  try {
+    options = JSON.parse(fs.readFileSync(configFileName, 'utf-8'));
+  } catch(err) {
+    Logger.warn('Could not get options for Prometheus Middleware.', err);
+  }
+
+  return expressPrometheusMiddleware(options);
+}