add metrics middleware to ogcard service

bskyogcard/package.json

···
     "@atproto/common": "^0.4.0",
     "@resvg/resvg-js": "^2.6.2",
     "express": "^4.19.2",
+    "express-prom-bundle": "^7.0.0",
     "http-terminator": "^3.2.0",
     "pino": "^9.2.0",
+    "prom-client": "^15.1.3",
     "react": "^18.3.1",
     "satori": "^0.10.13",
     "twemoji": "^14.0.2"
   },
   "devDependencies": {
+    "@types/express": "^4.17.21",
     "@types/node": "^20.14.3",
     "ts-node": "^10.9.2",
     "typescript": "^5.4.5"

bskyogcard/src/config.ts

···
 
 export type ServiceConfig = {
   port: number
+  metricsPort: number
   version?: string
   appviewUrl: string
   originVerify?: string
···
 
 export type Environment = {
   port?: number
+  metricsPort?: number
   version?: string
   appviewUrl?: string
   originVerify?: string
···
 export const readEnv = (): Environment => {
   return {
     port: envInt('CARD_PORT'),
+    metricsPort: envInt('CARD_METRICS_PORT'),
     version: envStr('CARD_VERSION'),
     appviewUrl: envStr('CARD_APPVIEW_URL'),
     originVerify: envStr('CARD_ORIGIN_VERIFY'),
···
 export const envToCfg = (env: Environment): Config => {
   const serviceCfg: ServiceConfig = {
     port: env.port ?? 3000,
+    metricsPort: env.metricsPort ?? 3001,
     version: env.version,
     appviewUrl: env.appviewUrl ?? 'https://api.bsky.app',
     originVerify: env.originVerify,

bskyogcard/src/index.ts

···
 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'
···
 
 export class CardService {
   public server?: http.Server
+  public metricsServer?: http.Server
   private terminator?: HttpTerminator
+  private metricsTerminator?: HttpTerminator
 
   constructor(
     public app: express.Application,
···
     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)
 
···
   }
 
   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()
   }
 }