feat: freeze typed metrics registry and bounded exemplars

crates/crank-metrics/tests/overhead.rs

@@ -0,0 +1,62 @@
+use std::{
+    hint::black_box,
+    time::{Duration, Instant},
+};
+
+use crank_metrics::{CacheOutcome, record_cache_outcome};
+use metrics_util::debugging::DebuggingRecorder;
+
+const SAMPLES: usize = 40;
+const WORK_UNITS: u64 = 500_000;
+
+#[test]
+fn reproducible_foundation_profile_stays_inside_latency_and_cpu_proxy_budgets() {
+    let recorder = DebuggingRecorder::new();
+    let snapshotter = recorder.snapshotter();
+    let mut baseline = Vec::with_capacity(SAMPLES);
+    let mut enabled = Vec::with_capacity(SAMPLES);
+
+    metrics::with_local_recorder(&recorder, || {
+        for index in 0..SAMPLES {
+            if index % 2 == 0 {
+                baseline.push(sample(false));
+                enabled.push(sample(true));
+            } else {
+                enabled.push(sample(true));
+                baseline.push(sample(false));
+            }
+        }
+    });
+
+    baseline.sort_unstable();
+    enabled.sort_unstable();
+    let baseline_p95 = baseline[SAMPLES * 95 / 100];
+    let enabled_p95 = enabled[SAMPLES * 95 / 100];
+    let baseline_cpu: Duration = baseline.iter().copied().sum();
+    let enabled_cpu: Duration = enabled.iter().copied().sum();
+    let series = snapshotter.snapshot().into_vec().len();
+
+    println!(
+        "metrics_profile_v1 samples={SAMPLES} work_units={WORK_UNITS} baseline_p95_ns={} enabled_p95_ns={} baseline_cpu_ns={} enabled_cpu_ns={} logical_series={series}",
+        baseline_p95.as_nanos(),
+        enabled_p95.as_nanos(),
+        baseline_cpu.as_nanos(),
+        enabled_cpu.as_nanos()
+    );
+    assert!(enabled_p95.as_nanos() * 100 <= baseline_p95.as_nanos() * 105);
+    assert!(enabled_cpu.as_nanos() * 100 <= baseline_cpu.as_nanos() * 110);
+    assert_eq!(series, 1);
+}
+
+fn sample(with_metrics: bool) -> Duration {
+    let started = Instant::now();
+    let mut value = 0x9e37_79b9_u64;
+    for index in 0..WORK_UNITS {
+        value = value.rotate_left(7) ^ index.wrapping_mul(0x100_0000_01b3);
+    }
+    black_box(value);
+    if with_metrics {
+        record_cache_outcome(CacheOutcome::Hit);
+    }
+    started.elapsed()
+}