build: split into multiple crates

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I6757cc99a0a5bc0c78193487df1ef52b6a6a6964

crates/metrics/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "ncro-metrics"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+prometheus.workspace = true

crates/metrics/src/lib.rs

@@ -0,0 +1,109 @@
+use std::sync::OnceLock;
+
+use prometheus::{
+  Encoder,
+  HistogramOpts,
+  HistogramVec,
+  IntCounter,
+  IntCounterVec,
+  IntGauge,
+  Opts,
+  Registry,
+  TextEncoder,
+};
+
+pub struct Metrics {
+  registry:                 Registry,
+  pub narinfo_cache_hits:   IntCounter,
+  pub narinfo_cache_misses: IntCounter,
+  pub narinfo_requests:     IntCounterVec,
+  pub nar_requests:         IntCounter,
+  pub upstream_race_wins:   IntCounterVec,
+  pub route_entries:        IntGauge,
+  pub upstream_latency:     HistogramVec,
+}
+
+static METRICS: OnceLock<Metrics> = OnceLock::new();
+
+#[expect(
+  clippy::expect_used,
+  reason = "metric names and labels are static constants validated during \
+            startup"
+)]
+pub fn get() -> &'static Metrics {
+  METRICS.get_or_init(|| {
+    let registry = Registry::new();
+    let narinfo_cache_hits = IntCounter::new(
+      "ncro_narinfo_cache_hits_total",
+      "Narinfo requests served from route cache.",
+    )
+    .expect("valid metric");
+    let narinfo_cache_misses = IntCounter::new(
+      "ncro_narinfo_cache_misses_total",
+      "Narinfo requests requiring upstream race.",
+    )
+    .expect("valid metric");
+    let narinfo_requests = IntCounterVec::new(
+      Opts::new("ncro_narinfo_requests_total", "Narinfo requests by status."),
+      &["status"],
+    )
+    .expect("valid metric");
+    let nar_requests =
+      IntCounter::new("ncro_nar_requests_total", "NAR streaming requests.")
+        .expect("valid metric");
+    let upstream_race_wins = IntCounterVec::new(
+      Opts::new(
+        "ncro_upstream_race_wins_total",
+        "Times each upstream won the narinfo race.",
+      ),
+      &["upstream"],
+    )
+    .expect("valid metric");
+    let route_entries = IntGauge::new(
+      "ncro_route_entries",
+      "Current number of route entries in SQLite.",
+    )
+    .expect("valid metric");
+    let upstream_latency = HistogramVec::new(
+      HistogramOpts::new(
+        "ncro_upstream_latency_seconds",
+        "Upstream narinfo race latency.",
+      ),
+      &["upstream"],
+    )
+    .expect("valid metric");
+
+    for collector in [
+      Box::new(narinfo_cache_hits.clone())
+        as Box<dyn prometheus::core::Collector>,
+      Box::new(narinfo_cache_misses.clone()),
+      Box::new(narinfo_requests.clone()),
+      Box::new(nar_requests.clone()),
+      Box::new(upstream_race_wins.clone()),
+      Box::new(route_entries.clone()),
+      Box::new(upstream_latency.clone()),
+    ] {
+      registry.register(collector).expect("register metric");
+    }
+
+    Metrics {
+      registry,
+      narinfo_cache_hits,
+      narinfo_cache_misses,
+      narinfo_requests,
+      nar_requests,
+      upstream_race_wins,
+      route_entries,
+      upstream_latency,
+    }
+  })
+}
+
+pub fn gather() -> String {
+  let mut buf = Vec::new();
+  let encoder = TextEncoder::new();
+  if encoder.encode(&get().registry.gather(), &mut buf).is_err() {
+    return String::new();
+  }
+  String::from_utf8_lossy(&buf).into_owned()
+}