Add Prometheus metrics export (issue 60)#81
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Issue #60. Adds an optional `metrics` cargo feature (off by default) that wires Prometheus-style observability into the matching engine without affecting matching latency or determinism. This commit lands the feature scaffolding plus the reject-counter surface; depth and trade counters land in the next commit. Surface - `metrics = "0.24"` is the new optional dependency. - `feature = "metrics"` enables it. - New `orderbook::metrics` module with feature-gated helpers that compile to no-ops when the feature is disabled, so call-sites in the matching hot path stay unconditional. - `orderbook_rejects_total{reason="..."}` counter — incremented exactly once per rejection. Hooked at the single chokepoint `OrderBook::track_state` (every `OrderStatus::Rejected` transition emits) plus the cold rejection sites that bypass `track_state` (lot-size, min/max range, IOC/FOK insufficient-liquidity, market order on empty book, STP cancel-taker). The label value is the `RejectReason::Display` string. Constraints respected - Allocation-free on the happy path. - No influence on matching outcomes. - `restore_from_snapshot_package` does not rehydrate metric counters. - `#[deny(unsafe_code)]` still on; no new unsafe.
Issue #60. Adds the remaining two operational metrics on top of the reject counter from the previous commit, plus the integration test binary that exercises all three. Surface - `orderbook_depth_levels_bid` / `orderbook_depth_levels_ask` — gauges, current count of distinct bid / ask price levels. Hooked via a new `OrderBook::record_depth_metric` helper that reads `self.bids.len()` / `self.asks.len()` (`SkipMap::len` is O(1)) and forwards to `metrics::record_depth`. Refreshed on every structural mutation: `place_order_in_book`, `add_order` resting insertion, cancel-driven level removal, modify-path level removal, matching empty-level cleanup, and `cancel_all_orders` bulk drain. - `orderbook_trades_total` — counter, incremented exactly once per emitted `MatchResult` transaction. Hooked at all three trade emission sites (`match_market_order_with_user`, `match_limit_order_with_user`, `add_order` matching path). The counter increments independently of whether a `TradeListener` is configured. Testing - New dedicated test binary `tests/metrics/` (gated on `required-features = ["metrics"]`) with three integration tests: reject + trade counters, depth gauges across add / cancel, and a determinism guard that confirms metrics emission produces byte-identical snapshots. - The metrics tests live in their own binary so the broader integration suite under `tests/unit/` doesn't perturb the global recorder via routine `OrderBook` mutations.
Issue #60. Adds a runnable example that demonstrates the optional `metrics` feature end-to-end: 1. Installs the `metrics-exporter-prometheus` recorder. 2. Builds an `OrderBook<()>`, seeds resting depth, crosses a few limit orders to print trades, then engages the kill switch and submits one order to force a reject. 3. Calls `PrometheusHandle::render()` and prints the exposition payload to stdout, showing all four metric series: `orderbook_rejects_total{reason="..."}`, `orderbook_trades_total`, `orderbook_depth_levels_bid`, `orderbook_depth_levels_ask`. The example is gated behind `required-features = ["metrics"]` and the examples crate gains a feature pass-through: metrics = ["orderbook-rs/metrics", "dep:metrics", "dep:metrics-exporter-prometheus"] `metrics = "0.24"` and `metrics-exporter-prometheus = "0.18"` are the new optional dev-dependencies on the examples crate. They do not affect the library crate. Run with: cd examples cargo run --features metrics --bin prometheus_export
Issue #60. Closes the documentation surface for the new optional `metrics` feature. - `CHANGELOG.md` gets a new "Added — Prometheus metrics feature (#60)" subsection under the unreleased `0.7.0` umbrella with the full surface (counters, gauges, label semantics) and the determinism / no-op-when-disabled guarantees. - `src/lib.rs` gains a "v0.7.0 — Metrics and Observability (#60)" block in the crate-level "What's New" section. Cross-references the helper module, the example binary, and the test binary.
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Added optional metrics feature to export Prometheus counters. See issue 60.