Fix: Always export server state metrics (is_banned, is_paused)

[dog-64]

Problem

The current implementation has a critical issue with gauge-type metrics like is_banned and is_paused:

Gauge metrics retain their last reported value indefinitely until explicitly updated. This means:

• If a server was banned (is_banned=1) and then information about it becomes unavailable
• The metric will continue showing is_banned=1 forever until explicitly set to 0
• This creates a false positive in monitoring: the metrics show the server is banned even after it's been unbanned

This is especially problematic because state metrics should always reflect the current actual state, not a stale cached value.

Solution

This PR refactors push_server_stats() to separate state and activity metrics:

• state_metrics (is_banned, is_paused) are now exported for every server on every metrics collection cycle

  • Ensures metrics always reflect current state
  • Guarantees stale values are immediately updated

• activity_metrics (bytes_received, bytes_sent, etc.) remain conditional

  • Only exported when server_info is available
  • Reduces noise for inactive servers

Impact

• Prometheus metrics now correctly reflect real-time server state
• No false positives from stale gauge values
• Monitoring/alerting based on is_banned and is_paused becomes reliable
• No breaking changes to metric format or API

Testing

• cargo check passes without errors
• Metrics endpoint functionality unchanged