feat(node): measure round-trip time to each peer #4763

Background

A node cannot tell how long its messages take to reach a peer:

• Packet::Ping goes out every 5 s on each outgoing connection and the receiver drops it (p2p.rs), so nothing comes back.
• A height update that arrives late looks the same as a peer whose indexer is behind (all_alive_participant_ids).

Between 5 and 11 Oct 2026, messages on some links queued for tens of seconds behind asset generation traffic, and we could only infer it from height lag. TCP statistics were set aside: they are of little use with our TDX setup and depend on each operator's host.

User Story

As a dev, I want the round-trip time of every peer link as a metric, so that I can tell a slow link from a slow peer.

Acceptance Criteria

Each node exports, per peer, the round-trip time of an echo that travels through the same queue and connections as MPC messages.

Resources & Additional Notes

• Implement echo connection check #1647 and feat: implement echo connection check with ping/pong mechanism #1687 tried an echo check. NetworkProtocolVersion now lets a node send it only to peers that support it.
• Related: Improve node connectivity metrics #4481, feat: Report network write progress before timing out #4491, [P2P Network] Add explicit reconnect requests when incoming connections are down. #4514.
• near/mpc-private#681, near/mpc-private#690.