add metrics middleware to ogcard service

bskyogcard/src/index.ts

@@ -2,7 +2,9 @@ import events from 'node:events'
 import type http from 'node:http'
 
 import express from 'express'
+import promBundle from 'express-prom-bundle'
 import {createHttpTerminator, type HttpTerminator} from 'http-terminator'
+import {register} from 'prom-client'
 
 import {type Config} from './config.js'
 import {AppContext} from './context.js'
@@ -13,7 +15,9 @@ export * from './logger.js'
 
 export class CardService {
   public server?: http.Server
+  public metricsServer?: http.Server
   private terminator?: HttpTerminator
+  private metricsTerminator?: HttpTerminator
 
   constructor(
     public app: express.Application,
@@ -24,6 +28,23 @@ export class CardService {
     let app = express()
 
     const ctx = await AppContext.fromConfig(cfg)
+
+    // Add Prometheus middleware for automatic HTTP instrumentation
+    const metricsMiddleware = promBundle({
+      includeMethod: true,
+      includePath: true,
+      includeStatusCode: true,
+      includeUp: true,
+      promClient: {
+        collectDefaultMetrics: {
+          timeout: 5000,
+        },
+      },
+      // Don't expose /metrics on main app - we'll use separate server
+      autoregister: false,
+    })
+    app.use(metricsMiddleware)
+
     app = routes(ctx, app)
     app.use(errorHandler)
 
@@ -31,14 +52,27 @@ export class CardService {
   }
 
   async start() {
+    // Start main application server
     this.server = this.app.listen(this.ctx.cfg.service.port)
     this.server.keepAliveTimeout = 90000
     this.terminator = createHttpTerminator({server: this.server})
     await events.once(this.server, 'listening')
+
+    // Start separate metrics server
+    const metricsApp = express()
+    metricsApp.get('/metrics', (_req, res) => {
+      res.set('Content-Type', register.contentType)
+      res.end(register.metrics())
+    })
+
+    this.metricsServer = metricsApp.listen(this.ctx.cfg.service.metricsPort)
+    this.metricsTerminator = createHttpTerminator({server: this.metricsServer})
+    await events.once(this.metricsServer, 'listening')
   }
 
   async destroy() {
     this.ctx.abortController.abort()
     await this.terminator?.terminate()
+    await this.metricsTerminator?.terminate()
   }
 }