[jonasz-lasut]

Closes the last recorded Go-parity gap (README "Differences from function-sdk-go", the ABI research verdict): a function built with this SDK now serves the same gRPC server metrics a function-sdk-go function serves, so platform dashboards and alerts work across Go and Rust functions unchanged.

What changed

• A private metrics module: grpc_server_started_total, grpc_server_handled_total (with grpc_code), grpc_server_msg_received_total, grpc_server_msg_sent_total - Confirmed (function-sdk-go v0.7.1 source): the exact names, label sets and help strings of grpcprometheus.NewServerMetrics() with default options, which is what the Go SDK installs.
• A tower layer around the whole router (so health Check is counted like Go's server-wide interceptor), reading the unary status from the grpc-status trailer or header; a client that disappears mid-call counts as Canceled.
• A /metrics endpoint beside the gRPC server: Args gains metrics_address (default :8080 - the Go SDK's DefaultMetricsAddress - METRICS_ADDRESS env, empty disables), mirroring how Serve() behaves in Go.
• Built on prometheus-client 0.25, per the recorded research verdict: the only purpose-built alternative, tonic-prometheus-layer, is pinned to tonic ^0.13 and would hold the SDK's tonic major back.

Deliberate parity choices

• Unary-only, because the Go SDK installs only metrics.UnaryServerInterceptor() - streaming methods (reflection, health Watch) were never counted there and are not counted here.
• Zero-initialization like metrics.InitializeMetrics(srv): every served method exists as a zero series from startup, streaming methods included (permanently zero).
• No grpc_server_handling_seconds: the Go SDK never enables the handling-time histogram.
• Recorded divergences: this SDK serves no v1beta1, so the v1beta1 series the Go SDK pre-created do not exist; and the exposition format is OpenMetrics 1.0 first (readable beyond Prometheus), with the classic text/plain; version=0.0.4 served to an Accept header that asks for it without accepting OpenMetrics - the series are what Go compatibility means, and both formats carry them identically. Registration compensates two prometheus-client behaviors verified in its source (it appends _total to counter samples and a trailing . to help strings), so the served text matches the Go SDK's byte for byte.

API surface

One new public field: Args::metrics_address. The module itself is private - no layer, registry or encoder types are exported, per the keep-serve-alone surface decision (#4). Breaking for Args struct literals (downstream: function-wasm constructs Args in tests and defines its own colliding --metrics-address flag - it drops that flag and adopts the SDK's on its next version bump), so this is a 0.3.0.

Testing

• The serve integration test now drives the full flow: one RunFunction + one health Check, then asserts on the served /metrics - started/received/handled-OK/sent all exactly 1 for RunFunction, the health Check counted, Watch present at zero, OpenMetrics content type + # EOF, and the classic rendering on an explicit text/plain Accept.
• Unit tests: grpc-go code-name mapping, the unary allowlist, format negotiation, the OpenMetrics/classic shape (single trailing help period included), and Canceled-on-drop without double-counting.
• cargo test --workspace green (6 suites); cargo clippy --workspace --all-targets -- -D warnings and cargo fmt --check clean.

Reference

• The layer is the same design proven in the function-wasm runtime (feat: serve the Go runtime gRPC server metrics jonasz-lasut/function-wasm#58 and #61, merged), adapted to this server's fixed service set. Deliberately not shared as a library: the runtime keeps its own copy per the minimal-surface decision - lift a common abstraction only when a second real consumer needs one.
• Author's note: implemented by Claude (Fable).