feat(metrics): add Prometheus-compatible metrics endpoint - #303

Summary

Implements a Prometheus-compatible metrics endpoint for monitoring SwiftChain Backend.

Changes

• Add a metrics endpoint that exposes application metrics in Prometheus format.
• Track HTTP request counts and request-duration histograms.
• Expose relevant process and infrastructure metrics, including memory usage, uptime, database connection-pool information, Redis health, circuit-breaker states, indexer lag, and Socket.IO metrics.
• Protect metrics access with authentication and administrator authorization.
• Add environment configuration for enabling metrics.
• Include unit tests for the metrics service and request-tracking middleware.
• Document the available metrics and deployment configuration.

Testing

• Add unit tests covering metrics collection and HTTP request instrumentation.
• Run the project's linting, formatting, and test checks before merging.

Closes #248