NestJS
Observed

@Injectable()
export class MetricsService {
  readonly registry = new Registry();

  readonly httpDuration = new Histogram({
    name: 'http_server_duration_seconds',
    help: 'HTTP request duration in seconds',
    labelNames: ['method', 'route', 'status'] as const,
    buckets: [0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5],
    registers: [this.registry],
  });

  constructor() {
    collectDefaultMetrics({ register: this.registry }); // CPU, memory, GC, event loop lag
  }
}

@Injectable()
export class HttpMetricsInterceptor implements NestInterceptor {
  constructor(private readonly metrics: MetricsService) {}

  intercept(ctx: ExecutionContext, next: CallHandler) {
    if (ctx.getType() !== 'http') return next.handle();
    const http = ctx.switchToHttp();
    const req = http.getRequest();
    const route = req.route?.path ?? 'unmatched'; // the template, never the raw URL
    const end = this.metrics.httpDuration.startTimer({ method: req.method, route });
    return next.handle().pipe(tap({
      next: () => end({ status: http.getResponse().statusCode }),
      error: (err) => end({ status: err?.getStatus?.() ?? 500 }),
    }));
  }
}

const perHistogram = 8 + 1 + 2; // buckets, +Inf, sum and count
const base = { routes: 40, methods: 3, statuses: 6, pods: 4 };
const count = (labels) => Object.values(labels).reduce((a, b) => a * b, perHistogram);

console.log('route, method, status, pod:', count(base).toLocaleString('en-US'));
console.log('plus a raw URL instead of the route:', count({ ...base, routes: 25000 }).toLocaleString('en-US'));
console.log('plus a userId label:', count({ ...base, users: 50000 }).toLocaleString('en-US'));

LoggerModule.forRoot({
  pinoHttp: {
    level: process.env.LOG_LEVEL ?? 'info',
    genReqId: (req, res) => {
      const id = req.headers['x-request-id'] ?? randomUUID();
      res.setHeader('x-request-id', id);
      return id;
    },
    redact: ['req.headers.authorization', 'req.headers.cookie', '*.password'],
    customProps: () => ({ service: 'game-api' }),
    transport: process.env.NODE_ENV !== 'production' ? { target: 'pino-pretty' } : undefined,
  },
}),

const app = await NestFactory.create(AppModule, { bufferLogs: true });
app.useLogger(app.get(Logger));

const sdk = new NodeSDK({
  resource: resourceFromAttributes({ [ATTR_SERVICE_NAME]: 'game-api' }),
  traceExporter: new OTLPTraceExporter(), // reads OTEL_EXPORTER_OTLP_ENDPOINT
  instrumentations: [getNodeAutoInstrumentations({
    '@opentelemetry/instrumentation-fs': { enabled: false },
  })],
});
sdk.start(); // must run before express, pg or ioredis are imported
process.on('SIGTERM', () => sdk.shutdown().finally(() => process.exit(0)));

async score(move: MoveDto) {
  return tracer.startActiveSpan('score-move', async (span) => {
    span.setAttributes({ 'game.id': move.gameId, 'word.length': move.word.length });
    try {
      return await this.computeScore(move);
    } catch (err) {
      span.recordException(err);
      span.setStatus({ code: SpanStatusCode.ERROR });
      throw err;
    } finally {
      span.end();
    }
  });
}

new Gauge({
  name: 'bullmq_jobs',
  help: 'Jobs by queue and state',
  labelNames: ['queue', 'state'] as const,
  registers: [registry],
  async collect() { // runs only when Prometheus scrapes
    const counts = await powerUps.getJobCounts('waiting', 'active', 'delayed', 'failed');
    for (const [state, n] of Object.entries(counts)) this.set({ queue: powerUps.name, state }, n);
  },
});

worker.on('completed', (job) =>
  jobDuration.observe({ queue: job.queueName, name: job.name }, (job.finishedOn! - job.processedOn!) / 1000));

handleConnection() { this.metrics.wsClients.inc({ namespace: '/game' }); }
handleDisconnect() { this.metrics.wsClients.dec({ namespace: '/game' }); }

groups:
  - name: game-api
    rules:
      - alert: HighErrorRate
        expr: sum(rate(http_server_duration_seconds_count{status=~"5.."}[5m])) / sum(rate(http_server_duration_seconds_count[5m])) > 0.02
        for: 10m
        labels:
          severity: page
        annotations:
          summary: "5xx above 2% for 10 minutes"
          runbook: "https://wiki.internal/runbooks/game-api-errors"