LoRaWAN unprocessable messages also contain unknown messages

[BobClaerhout]

The metric hono_telemetry_payload_bytes_count contains a status field. This field is either forwarded, unprocessable or undeliverable. We added prometheus alerts to follow up unprocessable messages in the lora adapter. However, in the lora adapter we are getting status unprocessable when the LNS is providing a message that is not an uplink message. This is completely different from an actual uplink message that is unprocessable because of any other unexpected exception. For this we would like to add an additional status: unknown, indicating that the message we received is an unknown message and thus not implemented which is fundamentally different from unrpocessable messages.