test(bff): /metrics exposes http-server request duration (refs #124)

services/bff/Bff.Tests/MetricsEndpointTests.cs

@@ -0,0 +1,28 @@
+using System.Net;
+using Microsoft.AspNetCore.Mvc.Testing;
+
+namespace Bff.Tests;
+
+/// <summary>
+/// S-16c (#124): the service exposes OTel HTTP-server metrics in Prometheus text format at /metrics,
+/// so Prometheus can scrape the golden signals (traffic, errors, latency) for the request path.
+/// </summary>
+public class MetricsEndpointTests(WebApplicationFactory<Program> factory)
+    : IClassFixture<WebApplicationFactory<Program>>
+{
+    [Fact]
+    public async Task Metrics_endpoint_exposes_http_server_request_duration_after_traffic()
+    {
+        var client = factory.CreateClient();
+
+        // One request produces an http.server.request.duration measurement...
+        await client.GetAsync("/health");
+
+        // ...which the /metrics scrape endpoint then exposes in Prometheus text format.
+        var response = await client.GetAsync("/metrics");
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        var body = await response.Content.ReadAsStringAsync();
+        Assert.Contains("http_server_request_duration", body);
+    }
+}