feat: add latency tracking for service import request-reply

src/NATS.Server/Imports/LatencyTracker.cs

@@ -0,0 +1,47 @@
+using System.Text.Json.Serialization;
+
+namespace NATS.Server.Imports;
+
+public sealed class ServiceLatencyMsg
+{
+    [JsonPropertyName("type")]
+    public string Type { get; set; } = "io.nats.server.metric.v1.service_latency";
+
+    [JsonPropertyName("requestor")]
+    public string Requestor { get; set; } = string.Empty;
+
+    [JsonPropertyName("responder")]
+    public string Responder { get; set; } = string.Empty;
+
+    [JsonPropertyName("status")]
+    public int Status { get; set; } = 200;
+
+    [JsonPropertyName("svc_latency")]
+    public long ServiceLatencyNanos { get; set; }
+
+    [JsonPropertyName("total_latency")]
+    public long TotalLatencyNanos { get; set; }
+}
+
+public static class LatencyTracker
+{
+    public static bool ShouldSample(ServiceLatency latency)
+    {
+        if (latency.SamplingPercentage <= 0) return false;
+        if (latency.SamplingPercentage >= 100) return true;
+        return Random.Shared.Next(100) < latency.SamplingPercentage;
+    }
+
+    public static ServiceLatencyMsg BuildLatencyMsg(
+        string requestor, string responder,
+        TimeSpan serviceLatency, TimeSpan totalLatency)
+    {
+        return new ServiceLatencyMsg
+        {
+            Requestor = requestor,
+            Responder = responder,
+            ServiceLatencyNanos = serviceLatency.Ticks * 100,
+            TotalLatencyNanos = totalLatency.Ticks * 100,
+        };
+    }
+}

tests/NATS.Server.Tests/ResponseRoutingTests.cs

@@ -124,4 +124,26 @@ public class ResponseRoutingTests
         resp2.To.ShouldBe("_INBOX.reply2");
         resp1.From.ShouldNotBe(resp2.From);
     }
+
+    [Fact]
+    public void LatencyTracker_should_sample_respects_percentage()
+    {
+        var latency = new ServiceLatency { SamplingPercentage = 0, Subject = "latency.test" };
+        LatencyTracker.ShouldSample(latency).ShouldBeFalse();
+
+        var latency100 = new ServiceLatency { SamplingPercentage = 100, Subject = "latency.test" };
+        LatencyTracker.ShouldSample(latency100).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void LatencyTracker_builds_latency_message()
+    {
+        var msg = LatencyTracker.BuildLatencyMsg("requester", "responder",
+            TimeSpan.FromMilliseconds(5), TimeSpan.FromMilliseconds(10));
+
+        msg.Requestor.ShouldBe("requester");
+        msg.Responder.ShouldBe("responder");
+        msg.ServiceLatencyNanos.ShouldBeGreaterThan(0);
+        msg.TotalLatencyNanos.ShouldBeGreaterThan(0);
+    }
 }