[Monitor] Add Prometheus metrics endpoint

[iAziz786]

Description

Basic metrics export

Acceptance Criteria

• Expose /metrics endpoint for Prometheus
• Include basic agent metrics (uptime, tasks, errors)
• Add HTTP request metrics
• Support custom metric registration
• Include system resource metrics

Implementation Hints

• Use prometheus/client_golang library
• Add standard metric types (counter, gauge, histogram)
• Include metric documentation

Dependencies

Health check endpoint