feature: add fastforward hello message latency metric

[Sunny6889]

What does this PR do?

Why are these changes required?

This PR has been tested by:

• Unit Tests
• Manual Testing

Follow up

Extra details

---

Summary by cubic

Adds a Prometheus histogram to measure handshake round‑trip latency on the SR<->FF path, from TCP connect to HelloMessage processed. This improves visibility into FastForward handshake performance.

• New Features
  • Introduced tron:handshake_latency_seconds and initialized it in MetricsHistogram.
  • Metric is recorded only for SR<->FF handshakes: inbound signed HelloMessage (witness or pqAuthSig) or peers listed in node.fastForward.nodes (via RelayService.isFastForwardPeer).

^{Written for commit ea3dc85. Summary will update on new commits. Review in cubic}

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ce31d6ec-cd39-4365-bb5e-13ef4cc61a79

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}