Feature: Add configurable interface and route metrics #62

Problem

Currently, interface and route metrics are hardcoded in the TUN implementation:

• Windows: Interface metric is set to 0 when AutoRoute is enabled
• Routes: Always use metric 0 (highest priority)

This creates issues when users need different routing priorities on different devices while using the same configuration file.

Proposed Solution

Add two new fields to the Options struct:

• InterfaceMetric uint32 - Sets the metric for the TUN interface (Windows only)
• RouteMetric uint32 - Sets the metric for routes through the TUN interface (Windows and Linux)

Behavior:

• InterfaceMetric = 0 (default):
  • When AutoRoute=true: metric is set to 0 (highest priority)
  • When AutoRoute=false: Windows automatic metric is used
• InterfaceMetric > 0: Explicitly specified metric (lower value = higher priority)
• RouteMetric = 0 (default): Metric 0 (highest priority)
• RouteMetric > 0: Explicitly specified metric
• On Linux, RouteMetric maps to the Priority field in netlink routes

Implementation

Changes required in:

• tun.go: Add InterfaceMetric and RouteMetric fields to Options
• tun_windows.go: Implement interface metric logic for IPv4/IPv6, update route metric
• tun_linux.go: Map RouteMetric to Priority field

Backward Compatibility

Fully backward compatible - when both fields are 0 (default), behavior remains unchanged.

Use Case

Users running the same configuration on multiple devices (e.g., desktop and mobile) can now set different routing priorities without maintaining separate config files.