feat(p2p): add metrics for actual peer count seen by PeerManager

[Drstone-23]

Closes #3011

What

Adds Functional_Peers_Connected and Reserved_Peers_Connected metrics that match GraphQL peer count.

Why

peer_count_per_protocol counts at libp2p transport level, while GraphQL returns peers from PeerManager (after LimitedBehaviour filtering). Not a bug, just different abstraction levels.

Changes

• New gauges in p2p_metrics.rs
• Auto-update on connect/disconnect
• Test validating the behavior

Note for infra

Grafana should use Functional_Peers_Connected instead of peer_count_per_protocol.

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[cursor]

PR Summary

Adds Functional_Peers_Connected and Reserved_Peers_Connected gauges, updates them on peer connect/disconnect, exposes count accessors, and adds tests (incl. functional vs discovery limit).

• Metrics:
  • Add gauges Functional_Peers_Connected and Reserved_Peers_Connected in crates/metrics/src/p2p_metrics.rs with setters and Prometheus registration.
  • Unit tests ensure presence and correct updates.
• P2P Service (crates/services/p2p/src/p2p_service.rs):
  • Update metrics on PeerConnected/PeerDisconnected via update_peer_metrics().
  • Wire in setters from p2p_metrics.
• PeerManager (crates/services/p2p/src/peer_manager.rs):
  • Expose non_reserved_peers_connected() and reserved_peers_connected() helpers.
• Tests (crates/services/p2p/src/p2p_service/tests.rs):
  • Add test verifying max_functional_peers_connected is enforced independently of discovery limit.
• Changelog:
  • Add .changes/added/3011.md describing new metrics and context.

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[fuel-cla-bot]

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