output csv instead of json on metrics endpoint

packages/delivery-service/src/metrics.ts

@@ -1,6 +1,8 @@
 import { DeliveryServiceProperties } from '@dm3-org/dm3-lib-delivery';
 import express from 'express';
 import { IDatabase } from './persistence/getDatabase';
+import { IntervalMetric } from './persistence/metrics/metricTypes';
+
 /**
  * The metrics endpoint returns the metrics of the delivery service.
  * These can include the number of messages received, the cumulative size of messages received, etc.,
@@ -13,9 +15,30 @@ export default (
 ) => {
     const router = express.Router();
     router.get('', async (req, res) => {
-        const metrics = await db.getMetrics(deliveryServiceProperties);
+        const metrics: IntervalMetric[] = await db.getMetrics(
+            deliveryServiceProperties,
+        );
+
+        if (metrics.length === 0) {
+            return res.status(204).send('No metrics data available');
+        }
+
+        // Convert metrics to CSV format
+        const headers = Object.keys(metrics[0]);
+        const csvRows = metrics.map((metric) =>
+            headers
+                .map((header) => metric[header as keyof IntervalMetric])
+                .join(','),
+        );
+
+        const csvData = [headers.join(','), ...csvRows].join('\n');
 
-        return res.status(200).send(JSON.stringify(metrics));
+        res.setHeader('Content-Type', 'text/csv');
+        res.setHeader(
+            'Content-Disposition',
+            'attachment; filename=metrics.csv',
+        );
+        return res.status(200).send(csvData);
     });
 
     return router;