add LoopbackInsinTelemetry with rate-limited warn logging

Inspectron.Epson.Tests/Telemetry/LoopbackInsinTelemetryTests.cs

@@ -0,0 +1,74 @@
+using System.Net;
+using System.Text;
+using System.Text.Json;
+using Inspectron.Epson.PrintServer.Telemetry;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace Inspectron.Epson.Tests.Telemetry;
+
+public class LoopbackInsinTelemetryTests
+{
+    private const int TestPort = 47823;
+
+    [Fact]
+    public async Task Emit_posts_json_payload_to_loopback()
+    {
+        using var listener = new HttpListener();
+        listener.Prefixes.Add($"http://127.0.0.1:{TestPort}/");
+        listener.Start();
+
+        using var http = new HttpClient();
+        var telemetry = new LoopbackInsinTelemetry(http, NullLogger.Instance);
+        telemetry.Emit("service.started", "version=1.0.14");
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3));
+        var ctxTask = listener.GetContextAsync();
+        var completed = await Task.WhenAny(ctxTask, Task.Delay(Timeout.Infinite, cts.Token));
+        Assert.Same(ctxTask, completed);
+        var ctx = ctxTask.Result;
+
+        Assert.Equal("POST", ctx.Request.HttpMethod);
+        Assert.Equal("/events", ctx.Request.Url!.AbsolutePath);
+        using var sr = new StreamReader(ctx.Request.InputStream, Encoding.UTF8);
+        var body = await sr.ReadToEndAsync();
+        using var doc = JsonDocument.Parse(body);
+        Assert.Equal("service.started", doc.RootElement.GetProperty("kind").GetString());
+        Assert.Equal("version=1.0.14",   doc.RootElement.GetProperty("message").GetString());
+        var at = doc.RootElement.GetProperty("at").GetString();
+        Assert.False(string.IsNullOrEmpty(at));
+        Assert.EndsWith("Z", at);
+
+        ctx.Response.StatusCode = 200;
+        ctx.Response.Close();
+        listener.Stop();
+    }
+
+    [Fact]
+    public async Task Emit_swallows_connection_refused_and_does_not_throw()
+    {
+        using var http = new HttpClient();
+        var telemetry = new LoopbackInsinTelemetry(http, NullLogger.Instance);
+        var ex = Record.Exception(() => telemetry.Emit("service.started", "version=1.0.14"));
+        Assert.Null(ex);
+        await Task.Delay(200);
+    }
+
+    [Fact]
+    public async Task Emit_swallows_500_response_and_does_not_throw()
+    {
+        using var listener = new HttpListener();
+        listener.Prefixes.Add($"http://127.0.0.1:{TestPort}/");
+        listener.Start();
+
+        using var http = new HttpClient();
+        var telemetry = new LoopbackInsinTelemetry(http, NullLogger.Instance);
+        telemetry.Emit("service.started", "version=1.0.14");
+
+        var ctx = await listener.GetContextAsync();
+        ctx.Response.StatusCode = 500;
+        ctx.Response.Close();
+
+        await Task.Delay(200);
+        listener.Stop();
+    }
+}