Add Prometheus metrics for performance monitoring

[ways]

• Added prometheus-client dependency
• Instrumented location lookup with timing (cached vs uncached)
• Instrumented weather data fetch timing
• Instrumented meteogram formatting timing
• Added overall response time tracking
• Added request counter with status labels (success, not_found, blacklisted, help, error_no_service)
• Metrics endpoint exposed on port 8000 by default (configurable with --metrics-port)
• Updated README with metrics documentation

Available metrics:

• fingr_requests_total - Total requests by status
• fingr_location_lookup_seconds - Location lookup timing
• fingr_weather_fetch_seconds - Weather API fetch timing
• fingr_meteogram_format_seconds - Meteogram rendering timing
• fingr_response_seconds - Total response timing