feat: add service export latency tracking with p50/p90/p99 (Gap 9.1)

Add ServiceLatencyTracker with sorted-sample histogram, percentile getters (p50/p90/p99), average/min/max, reset, and immutable snapshot. Wire LatencyTracker and RecordServiceLatency onto Account. Cover with 11 xUnit tests.

tests/NATS.Server.Tests/Auth/ServiceLatencyTrackerTests.cs

@@ -0,0 +1,168 @@
+// Tests for service export latency tracker with p50/p90/p99 percentile histogram.
+// Go reference: accounts_test.go TestServiceLatency, serviceExportLatencyStats.
+
+using NATS.Server.Auth;
+
+namespace NATS.Server.Tests.Auth;
+
+public class ServiceLatencyTrackerTests
+{
+    [Fact]
+    public void RecordLatency_IncrementsTotalRequests()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        tracker.RecordLatency(10.0);
+        tracker.RecordLatency(20.0);
+        tracker.RecordLatency(30.0);
+
+        tracker.TotalRequests.ShouldBe(3L);
+    }
+
+    [Fact]
+    public void GetP50_ReturnsMedian()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        foreach (var v in new double[] { 1, 2, 3, 4, 5 })
+            tracker.RecordLatency(v);
+
+        // Sorted: [1, 2, 3, 4, 5], index = (int)(0.50 * 4) = 2 → value 3
+        tracker.GetP50().ShouldBe(3.0);
+    }
+
+    [Fact]
+    public void GetP90_ReturnsHighPercentile()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        for (var i = 1; i <= 100; i++)
+            tracker.RecordLatency(i);
+
+        // Sorted [1..100], index = (int)(0.90 * 99) = (int)89.1 = 89 → value 90
+        tracker.GetP90().ShouldBe(90.0);
+    }
+
+    [Fact]
+    public void GetP99_ReturnsTopPercentile()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        for (var i = 1; i <= 100; i++)
+            tracker.RecordLatency(i);
+
+        // Sorted [1..100], index = (int)(0.99 * 99) = (int)98.01 = 98 → value 99
+        tracker.GetP99().ShouldBe(99.0);
+    }
+
+    [Fact]
+    public void AverageLatencyMs_CalculatesCorrectly()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        tracker.RecordLatency(10.0);
+        tracker.RecordLatency(20.0);
+        tracker.RecordLatency(30.0);
+
+        tracker.AverageLatencyMs.ShouldBe(20.0);
+    }
+
+    [Fact]
+    public void MinLatencyMs_ReturnsMinimum()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        tracker.RecordLatency(15.0);
+        tracker.RecordLatency(5.0);
+        tracker.RecordLatency(10.0);
+
+        tracker.MinLatencyMs.ShouldBe(5.0);
+    }
+
+    [Fact]
+    public void MaxLatencyMs_ReturnsMaximum()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        tracker.RecordLatency(5.0);
+        tracker.RecordLatency(15.0);
+        tracker.RecordLatency(10.0);
+
+        tracker.MaxLatencyMs.ShouldBe(15.0);
+    }
+
+    [Fact]
+    public void Reset_ClearsSamples()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        tracker.RecordLatency(10.0);
+        tracker.RecordLatency(20.0);
+        tracker.SampleCount.ShouldBe(2);
+        tracker.TotalRequests.ShouldBe(2L);
+
+        tracker.Reset();
+
+        tracker.SampleCount.ShouldBe(0);
+        tracker.TotalRequests.ShouldBe(0L);
+        tracker.AverageLatencyMs.ShouldBe(0.0);
+        tracker.MinLatencyMs.ShouldBe(0.0);
+        tracker.MaxLatencyMs.ShouldBe(0.0);
+        tracker.GetP50().ShouldBe(0.0);
+    }
+
+    [Fact]
+    public void GetSnapshot_ReturnsImmutableSnapshot()
+    {
+        var tracker = new ServiceLatencyTracker();
+
+        tracker.RecordLatency(10.0);
+        tracker.RecordLatency(20.0);
+        tracker.RecordLatency(30.0);
+
+        var snapshot = tracker.GetSnapshot();
+
+        snapshot.TotalRequests.ShouldBe(3L);
+        snapshot.SampleCount.ShouldBe(3);
+        snapshot.AverageMs.ShouldBe(20.0);
+        snapshot.MinMs.ShouldBe(10.0);
+        snapshot.MaxMs.ShouldBe(30.0);
+        // P50 of [10, 20, 30]: index = (int)(0.50 * 2) = 1 → 20
+        snapshot.P50Ms.ShouldBe(20.0);
+
+        // Mutating tracker after snapshot does not change the snapshot
+        tracker.RecordLatency(1000.0);
+        snapshot.MaxMs.ShouldBe(30.0);
+        snapshot.SampleCount.ShouldBe(3);
+    }
+
+    [Fact]
+    public void MaxSamples_EvictsOldest()
+    {
+        var tracker = new ServiceLatencyTracker(maxSamples: 5);
+
+        for (var i = 1; i <= 10; i++)
+            tracker.RecordLatency(i);
+
+        // Only the last 5 samples should remain (6, 7, 8, 9, 10)
+        tracker.SampleCount.ShouldBe(5);
+        // TotalRequests counts all recorded calls, not just retained ones
+        tracker.TotalRequests.ShouldBe(10L);
+        // Minimum of retained samples is 6
+        tracker.MinLatencyMs.ShouldBe(6.0);
+        // Maximum of retained samples is 10
+        tracker.MaxLatencyMs.ShouldBe(10.0);
+    }
+
+    [Fact]
+    public void Account_RecordServiceLatency_DelegatesToTracker()
+    {
+        var account = new Account("test");
+
+        account.RecordServiceLatency(50.0);
+        account.RecordServiceLatency(100.0);
+
+        account.LatencyTracker.TotalRequests.ShouldBe(2L);
+        account.LatencyTracker.AverageLatencyMs.ShouldBe(75.0);
+    }
+}