Prometheus metrics for request latency

                  import client from 'prom-client';
import type { RequestHandler } from 'express';

export const httpDuration = new client.Histogram({
  name: 'http_request_duration_seconds',
  help: 'HTTP request duration in seconds',
  labelNames: ['method', 'route', 'status'] as const,
  buckets: [0.01, 0.05, 0.1, 0.25, 0.5, 1, 2, 5]
});

export const metricsMiddleware: RequestHandler = (req, res, next) => {
  const end = httpDuration.startTimer();
  res.on('finish', () => {
    const route = (req.route && req.route.path) ? String(req.route.path) : req.path;
    end({ method: req.method, route, status: String(res.statusCode) });
  });
  next();
};

export async function metricsHandler(_req: any, res: any) {
  res.setHeader('Content-Type', client.register.contentType);
  res.send(await client.register.metrics());
}