fix(metrics-auth): DisableLogin no longer opens /metrics; log correlation pair; host-level tests

- The metrics scheme skips its dashboard-cookie fallback while
  Dashboard:DisableLogin is on, so the dev auto-login handler cannot
  satisfy RequireAuthentication=true.
- Invoke logs client_correlation_id <-> worker envelope id once at Debug.
- MetricsEndpointAuthenticationTests drives the real pipeline: anonymous
  loopback 401 (no /login redirect), Viewer cookie 200, admin key 200,
  non-admin key 403, DisableLogin 401, /health/active 200.
- Docs: DisableLogin behaviour; the /metrics limiter partition is per IP
  vs gRPC per connection, so only the per-key aggregate is shared.

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -148,6 +148,19 @@ public sealed class MxAccessGatewayService(
             // the same value the constraint-denied replies carry — so a client can match every
             // reply to its request. The worker reply carries the gateway's internal envelope id,
             // which is a gateway↔worker routing detail and is overwritten here, empty included.
+            // Record the pairing once, before the envelope id is overwritten, so a client error
+            // report quoting its correlation id can be matched to gateway and worker log lines
+            // (which carry the envelope id). Ids and the command kind only — never values.
+            if (logger.IsEnabled(LogLevel.Debug))
+            {
+                logger.LogDebug(
+                    "Invoke {CommandKind} for session {SessionId}: client correlation id {ClientCorrelationId} maps to worker envelope correlation id {WorkerCorrelationId}.",
+                    command.Kind,
+                    request.SessionId,
+                    request.ClientCorrelationId,
+                    publicReply.CorrelationId);
+            }
+
             publicReply.CorrelationId = request.ClientCorrelationId;
 
             session.TrackCommandReply(commandToInvoke, publicReply);