chore(cbind): nim-ffi migration [8/9] — relay, peerstore, metrics (API parity)

[gmelodie]

nim-ffi cbind migration — PR train

1. chore(cbind): nim-ffi migration [1/9] — deps + pinning scaffold #2772 — deps + pinning
2. chore(cbind): nim-ffi migration [2/9] — new-library scaffold + CI #2773 — scaffold + CI
3. chore(cbind): nim-ffi migration [3/9] — connectivity & identity #2774 — connectivity
4. chore(cbind): nim-ffi migration [4/9] — streams & custom protocols (+ echo example) #2775 — streams (+ echo)
5. chore(cbind): nim-ffi migration [5/9] — pubsub / gossipsub (+ gossipsub example) #2776 — pubsub (+ gossipsub)
6. chore(cbind): nim-ffi migration [6/9] — kademlia + CID + crypto #2777 — kad + CID + crypto
7. chore(cbind): nim-ffi migration [7/9] — service discovery + extended peer records #2778 — service discovery
8. chore(cbind): nim-ffi migration [8/9] — relay, peerstore, metrics (API parity) #2779 — relay / peerstore / metrics ← this PR
9. chore(cbind): nim-ffi migration [9/9] — flip to libp2p.nim, delete legacy cbind #2780 — flip + delete legacy

---

Part 8 of a 9-PR train that replaces the hand-rolled cbind/ C bindings with the generated nim-ffi codegen, landing it domain by domain so each PR stays reviewable and CI-green (build the FFI lib + generate bindings + build/run examples).

Targets chore/cbind/ffi/07-service-disco (stacked).

This PR adds

• libp2pCircuitRelayReserve.
• Peerstore: addPeer, deletePeer, getPeerInfo, getPeers, setPeerAddresses, setPeerProtocols.
• libp2pCollectMetrics (OpenMetrics -> JSON).
• After this PR the new library is at full parity with the legacy cbind API surface.

Review artifact: Header diff shows the relay/peerstore/metrics entry points — the last additions before the flip.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.15%. Comparing base (c848748) to head (e09e4dd).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           chore/cbind/ffi/07-service-disco    #2779      +/-   ##
====================================================================
- Coverage                             81.15%   81.15%   -0.01%     
====================================================================
  Files                                   170      170              
  Lines                                 31027    31027              
  Branches                                 12       13       +1     
====================================================================
- Hits                                  25181    25180       -1     
- Misses                                 5846     5847       +1     

see 3 files with indirect coverage changes

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