feat: add MonitorServer with /healthz and /varz endpoints

src/NATS.Server/Monitoring/MonitorServer.cs

@@ -0,0 +1,62 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Logging;
+
+namespace NATS.Server.Monitoring;
+
+/// <summary>
+/// HTTP monitoring server providing /healthz, /varz, and other monitoring endpoints.
+/// Corresponds to Go server/monitor.go HTTP server setup.
+/// </summary>
+public sealed class MonitorServer : IAsyncDisposable
+{
+    private readonly WebApplication _app;
+    private readonly ILogger<MonitorServer> _logger;
+
+    public MonitorServer(NatsServer server, NatsOptions options, ILoggerFactory loggerFactory)
+    {
+        _logger = loggerFactory.CreateLogger<MonitorServer>();
+
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.WebHost.UseUrls($"http://{options.MonitorHost}:{options.MonitorPort}");
+        builder.Logging.ClearProviders();
+
+        _app = builder.Build();
+        var basePath = options.MonitorBasePath ?? "";
+
+        var varzHandler = new VarzHandler(server, options);
+
+        _app.MapGet(basePath + "/", () => Results.Ok(new
+        {
+            endpoints = new[]
+            {
+                "/varz", "/connz", "/healthz", "/routez",
+                "/gatewayz", "/leafz", "/subz", "/accountz", "/jsz",
+            },
+        }));
+        _app.MapGet(basePath + "/healthz", () => Results.Ok("ok"));
+        _app.MapGet(basePath + "/varz", async () => Results.Ok(await varzHandler.HandleVarzAsync()));
+
+        // Stubs for unimplemented endpoints
+        _app.MapGet(basePath + "/routez", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/gatewayz", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/leafz", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/subz", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/subscriptionsz", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/accountz", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/accstatz", () => Results.Ok(new { }));
+        _app.MapGet(basePath + "/jsz", () => Results.Ok(new { }));
+    }
+
+    public async Task StartAsync(CancellationToken ct)
+    {
+        await _app.StartAsync(ct);
+        _logger.LogInformation("Monitoring listening on {Urls}", string.Join(", ", _app.Urls));
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _app.DisposeAsync();
+    }
+}