p2p: cache per-message ingress gauges to avoid allocs in Peer.handle

[AskAlexSharov]

Problem

Peer.handle runs in the per-peer read loop for every inbound subprotocol message. With metrics enabled it did, per message:

m := fmt.Sprintf("%s_%s_%d_%#02x", ingressMeterName, proto.Name, proto.Version, msg.Code-proto.offset)
metrics.GetOrCreateGauge(m).SetUint32(msg.meterSize)
metrics.GetOrCreateGauge(m + "_packets").Set(1)

That allocated, per message: the Sprintf string, the + "_packets" string, two &gauge{} wrappers from GetOrCreateGauge, and the variadic arg boxing — and each GetOrCreateGauge also took a mutex + map lookup. The gauge identity only depends on (proto.Name, proto.Version, relativeCode), all stable for the lifetime of a protoRW.

Change

Cache the gauges per protoRW, indexed by the protocol-relative message code. The first message for a code creates and caches the gauges (preserving the original lazy registration — no zero-valued metrics for codes that never arrive); subsequent messages just index the slice and call Set.

Behavior is identical: same gauge names, same values. Safe without locking because handle only runs in the single per-peer read-loop goroutine and each peer owns its own protoRW instances (confirmed under -race).

Numbers

Benchmarked on the actual code path (benchmem, Apple M4 Max):

	allocs/op	bytes/op	ns/op
Before	5	128 B	~152 ns
After (cache warm)	0	0 B	~3.3 ns

5 allocations → 0 per inbound message, ~46× faster per call.

Tests

• TestProtoRWMeterIngress — verifies the byte/packet gauges still receive the correct values.
• TestProtoRWMeterIngressNoAllocs — asserts 0 allocs/op after warmup (regression guard).

[Copilot]

Pull request overview

This PR aims to reduce per-inbound-message overhead in Peer.handle when metrics are enabled by caching ingress byte/packet gauges on protoRW and updating them via a helper (meterIngress) rather than performing per-message metric lookup/registration work.

Changes:

• Replace direct per-message gauge updates in Peer.handle with a protoRW.meterIngress helper.
• Add protoRW.meterIngress to lazily allocate/cache per-message-code ingress gauges.
• Add tests verifying gauge values and asserting zero allocations after cache warm-up.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
p2p/peer.go	Routes ingress metering through protoRW.meterIngress and adds the caching/lazy-init helper.
p2p/peer_test.go	Adds correctness + allocation regression tests for meterIngress.

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