add discovery middleware for websocket

[proppy]

It would be nice if we implemented websocket external IP discovery through metadata as a middleware.

[proppy]

See:
https://github.com/proppy/cacophon/blob/master/frontend/api/services/DiscoveryService.js
and:
https://github.com/proppy/cacophon/blob/master/frontend/api/services/MetadataService.js