feat: add Prometheus /metrics endpoint

[saidsef]

🩺 Problem Statement

The application currently lacks a standardized endpoint for exposing runtime metrics, making it difficult to monitor health, performance, and usage in production environments. This limits observability and proactive troubleshooting.

📝 Description

Integrate a Prometheus-compatible /metrics endpoint using the prometheus_client Python library. The endpoint should expose key application metrics such as request count, latency, active requests, and response size. The metrics server should run on a configurable port and be accessible for Prometheus scraping. Implementation should be robust, well-documented, and follow best practices for production monitoring.

🎯 Goals

• Add /metrics endpoint using prometheus_client.
• Track and expose:
  • Total request count
  • Request latency (histogram)
  • Number of active requests
  • Response size (summary)
• Make metrics server port configurable via environment variable.
• Ensure endpoint is accessible and compatible with Prometheus scraping.
• Document usage, configuration, and integration steps.

✅ Expected Result

• /metrics endpoint available and serving Prometheus metrics.
• Metrics are updated in real-time and reflect application activity.
• Documentation is clear and enables easy integration with Prometheus/Grafana.
• Improved observability and monitoring for the application.

📚 Resources

• prometheus_client documentation
• Prometheus metrics best practices
• Python Prometheus examples
• Prometheus monitoring overview