Can a Kovri network observer distinguish Monero traffic from non network traffic? If no how could the success of a campaign to increase non Monero Kovri usage be measured?
The kovri traffic is hidden by i2p protocol.
This protocol hide the final destination to each node in the path to this destination.
So there is technically no way to tell if the traffic is monero traffic or anything else.
That's the beauty of i2p.
To be more technical: garlic routing and garlic encryption take care of encrypting your message in the same layered-deepness than the nodes you should go through within the tunnel to your destination.
You will use successively each node public key to encrypt the message layer by layer so that each node on the path will use its private key to decrypt the outer layer and know the next node to pass inner message to, and so on until the final destination.
There is no way to measure the non-monero usage of kovri.