feat: track protocol stream open/close count in metrics

[Patrick-Ehimen]

Summary

• Adds two new counter metrics to track stream lifecycle events:
  • libp2p_protocol_streams_opened_total - incremented when a stream is opened
  • libp2p_protocol_streams_closed_total - incremented when a stream is closed
• Uses the same label format as the existing gauge: {direction} {protocol}
• Preserves the existing libp2p_protocol_streams_total gauge for backward compatibility

Motivation

The existing gauge metric only shows current state. These counters enable:

• Calculating stream open/close rates using Prometheus rate() or increase()
• Analyzing stream throughput patterns over time
• Deriving current streams as opened_total - closed_total

Test plan

• All existing connection tests pass (20/20)
• All existing connection-manager tests pass (18/18)
• Build succeeds

Fixes #3262