[BugenZhao]

Overview

#56939 added Prometheus metrics for the P2P MooncakeConnector. Under the Rust frontend these stats are dropped: the payload matches no typed KvConnectorStats variant, decodes as an unknown-child Multi, and records nothing. This PR decodes the payload and exports the six vllm:mooncake_* families with Python's names, labels, and buckets.

Claims

• With a MooncakeConnector (bare or as a MultiConnector child), /metrics reports vllm:mooncake_xfer_time_seconds, vllm:mooncake_bytes_transferred, vllm:mooncake_num_descriptors (histograms) and vllm:mooncake_num_failed_transfers, vllm:mooncake_num_failed_recvs, vllm:mooncake_num_kv_expired_reqs (counters), labeled model_name and engine, as Python's MooncakePromMetrics does.
• NIXL and Mooncake store metrics are unchanged.

Validation

Commands from .buildkite/scripts/run-rust-frontend-cargo-ci.sh (style-clippy and test), run from the repo root:

cargo fmt --manifest-path rust/Cargo.toml --all -- --check   # ok
cargo sort --workspace --check rust                          # ok
cargo deny --manifest-path rust/Cargo.toml --config rust/deny.toml check bans   # ok
cargo clippy --manifest-path rust/Cargo.toml --workspace --all-targets --all-features --locked -- -D warnings   # ok
cargo nextest run --manifest-path rust/Cargo.toml --workspace --all-features --locked --no-fail-fast
# 2295 tests run: 2295 passed, 13 skipped

• metrics::tests::mooncake_transfer_stats_are_recorded_into_mooncake_metrics: a Python-shaped MooncakeConnector payload goes through decode and recording; the exposition is snapshotted.
• metrics::tests::kv_connector_stats_are_recorded_into_mooncake_and_nixl_metrics (extended): a MultiConnector payload with a MooncakeConnector child records into the same families.
• tests::client::python_msgpack_fixtures_match_rust_encoding: the Python fixture script now msgpack-encodes a MooncakeConnector payload, bare and as a MultiConnector child, and Rust decodes both into the new types.
• With the variant placed after Multi (the old decode behavior), the bare-payload and fixture tests fail; without the MultiConnector recording branch, the extended test fails.

Details

• Decode (engine-core-client protocol/stats.rs): MooncakeTransferStats mirrors MooncakeKVConnectorStats.data with deny_unknown_fields. It is its own KvConnectorStats variant, placed before Multi (which accepts any map; the enum now documents this order contract), and the MooncakeConnector child of MultiConnectorStats.
• Register (vllm-metrics SchedulerMetrics): the six families, reusing the NIXL bucket constructors, which match Python's buckets.
• Record (engine-core-client metrics.rs): per engine, next to record_nixl_stats.

This is the NIXL path with Mooncake's field set (no post_duration; num_failed_recvs instead of num_failed_notifications), with no new abstraction.

Difference kept: like the existing NIXL families in Rust, the Mooncake series are created for every engine at startup and report 0 when no Mooncake connector is configured; Python registers connector families only for the configured connector.

Not a duplicate: no open PR adds these families to Rust. #60856 (inactive since 2026-10-09) decodes NIXL connector stats in the same files and does not touch Mooncake.

AI assistance (Claude Code) was used to prepare this change; the human submitter reviewed every changed line.

🤖 Generated with Claude Code

https://claude.ai/code/session_012jJ3QaARmRe3jZJYTfHsCB

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Pull Request Checklist

• I used vLLM's /pr-checklist skill. (Mandatory for agents, optional for humans).

• AI assistance was used during the creation of this PR.

• Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.

• Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.

• Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.

• Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.

[claude]

Claude Code Review

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[BugenZhao]

/ci run

[github-actions]

✅ Triggered Buildkite CI #96858 for commit 4c4abd4d0e27.

ℹ️ This PR is 1 commit behind upstream main. Merged upstream main into this branch as 4c4abd4d0e27 before starting CI. Pull the branch before pushing more changes.