How do you implement a custom observability stack in Node.js?

Question

Accepted Answer

**Observability** — metrics, logs, and traces.

```javascript
const promClient = require("prom-client");
const pino = require("pino");
const { NodeTracerProvider } = require("@opentelemetry/sdk-trace-node");

// Metrics
const metrics = {
    httpRequestDuration: new promClient.Histogram({
        name: "http_request_duration_seconds",
        help: "Duration of HTTP requests",
        labelNames: ["method", "route", "status"],
        buckets: [0.1, 0.5, 1, 2, 5],
    }),
    activeConnections: new promClient.Gauge({
        name: "active_connections",
        help: "Number of active connections",
    }),
    errorRate: new promClient.Counter({
        name: "errors_total",
        help: "Total errors",
        labelNames: ["type"],
    }),
};

// Logging
const logger = pino({
    level: "info",
    formatters: {
        level: (label) => ({ level: label }),
    },
});

// Middleware
app.use((req, res, next) => {
    const start = Date.now();
    metrics.activeConnections.inc();

res.on("finish", () => {
        const duration = (Date.now() - start) / 1000;
        metrics.httpRequestDuration
            .labels(req.method, req.route?.path || "unknown", res.statusCode)
            .observe(duration);
        metrics.activeConnections.dec();

logger.info({
            method: req.method,
            url: req.url,
            status: res.statusCode,
            duration,
        });
    });

next();
});

// Metrics endpoint
app.get("/metrics", async (req, res) => {
    res.setHeader("Content-Type", promClient.register.contentType);
    res.end(await promClient.register.metrics());
});
```

Node.js: How do you implement a custom observability stack in Node.js?