feat: add per-subsystem log control via --log_level_override CLI flag

Adds LogOverrides property to NatsOptions and a --log_level_override=namespace=level CLI flag that wires Serilog MinimumLevel.Override entries so operators can tune verbosity per .NET namespace without changing the global log level.

src/NATS.Server.Host/Program.cs

@@ -85,6 +85,14 @@ for (int i = 0; i < args.Length; i++)
         case "--service":
             windowsService = true;
             break;
+        case "--log_level_override" when i + 1 < args.Length:
+            var parts = args[++i].Split('=', 2);
+            if (parts.Length == 2)
+            {
+                options.LogOverrides ??= new();
+                options.LogOverrides[parts[0]] = parts[1];
+            }
+            break;
     }
 }
 
@@ -135,6 +143,16 @@ else if (options.Syslog)
     logConfig.WriteTo.LocalSyslog("nats-server");
 }
 
+// Apply per-subsystem log level overrides
+if (options.LogOverrides is not null)
+{
+    foreach (var (ns, level) in options.LogOverrides)
+    {
+        if (Enum.TryParse<Serilog.Events.LogEventLevel>(level, true, out var serilogLevel))
+            logConfig.MinimumLevel.Override(ns, serilogLevel);
+    }
+}
+
 Log.Logger = logConfig.CreateLogger();
 
 if (windowsService)

src/NATS.Server/NatsOptions.cs

@@ -90,5 +90,8 @@ public sealed class NatsOptions
     public OcspConfig? OcspConfig { get; set; }
     public bool OcspPeerVerify { get; set; }
 
+    // Per-subsystem log level overrides (namespace -> level)
+    public Dictionary<string, string>? LogOverrides { get; set; }
+
     public bool HasTls => TlsCert != null && TlsKey != null;
 }

tests/NATS.Server.Tests/NatsOptionsTests.cs

@@ -15,3 +15,24 @@ public class NatsOptionsTests
         opts.Tags.ShouldBeNull();
     }
 }
+
+public class LogOverrideTests
+{
+    [Fact]
+    public void LogOverrides_defaults_to_null()
+    {
+        var options = new NatsOptions();
+        options.LogOverrides.ShouldBeNull();
+    }
+
+    [Fact]
+    public void LogOverrides_can_be_set()
+    {
+        var options = new NatsOptions
+        {
+            LogOverrides = new() { ["NATS.Server.Protocol"] = "Trace" }
+        };
+        options.LogOverrides.ShouldNotBeNull();
+        options.LogOverrides["NATS.Server.Protocol"].ShouldBe("Trace");
+    }
+}