Determine if MQTT-based metrics are available to be exposed

### What motivated this proposal?

For users leveraging the MQTT interface in the server, it would be useful to have metrics exposed indicating the traffic wrt MQTT clients.

### What is the proposed change?

Exposing additional metrics if available.

### Who benefits from this change?

Anyone using MQTT clients with the NATS server.

### What alternatives have you evaluated?

_No response_