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',